MIME types

MIME types (IANA media types)

A media type (also known as a Multipurpose Internet Mail Extensions or MIME type) is a standard that indicates the nature and format of a document, file, or assortment of bytes. It is defined and standardized in IETF's RFC 6838.

The Internet Assigned Numbers Authority (IANA) is responsible for all official MIME types, and you can find the most up-to-date and complete list at their Media Types page.

Structure of a MIME type

The simplest MIME type consists of a type and a subtype; these are each strings which, when concatenated with a slash (/) between them, comprise a MIME type. No whitespace is allowed in a MIME type:

type/subtype

The type represents the general category into which the data type falls, such as video or text. The subtype identifies the exact kind of data of the specified type the MIME type represents. For example, for the MIME type text, the subtype might be plain (plain text), html (HTML source code), or calendar (for iCalendar/.ics) files.

Each type has its own set of possible subtypes, and a MIME type always has both a type and a subtype, never just one or the other.

An optional parameter can be added to provide additional details:

type/subtype;parameter=value

For example, for any MIME type whose main type is text, the optional charset parameter can be used to specify the character set used for the characters in the data. If no charset is specified, the default is ASCII (US-ASCII) unless overridden by the user agent's settings. To specify a UTF-8 text file, the MIME type text/plain;charset=UTF-8 is used.

MIME types are case-insensitive but are traditionally written in lowercase, with the exception of parameter values, whose case may or may not have specific meaning.

Types

There are two classes of type: discrete and multipart. Discrete types are types which represent a single file or medium, such as a single text or music file, or a single video. A multipart type is one which represents a document that's comprised of multiple component parts, each of which may have its own individual MIME type; or, a multipart type may encapsulate multiple files being sent together in one transaction. For example, multipart MIME types are used when attaching multiple files to an email.

Discrete types

The discrete types currently registered with the IANA are:

application

Any kind of binary data that doesn't fall explicitly into one of the other types; either data that will be executed or interpreted in some way or binary data that requires a specific application or category of application to use. Generic binary data (or binary data whose true type is unknown) is application/octet-stream. Other common examples include application/pdf, application/pkcs8, and application/zip. (Registration at IANA)

audio

Audio or music data. Examples include audio/mpeg, audio/vorbis. (Registration at IANA)

example

Reserved for use as a placeholder in examples showing how to use MIME types. These should never be used outside of sample code listings and documentation. example can also be used as a subtype; for instance, in an example related to working with audio on the web, the MIME type audio/example can be used to indicate that the type is a placeholder and should be replaced with an appropriate one when using the code in the real world.

font

Font/typeface data. Common examples include font/woff, font/ttf, and font/otf. (Registration at IANA)

image

Image or graphical data including both bitmap and vector still images as well as animated versions of still image formats such as animated GIF or APNG. Common examples are image/jpeg, image/png, and image/svg+xml. (Registration at IANA)

model

Model data for a 3D object or scene. Examples include model/3mf and model/vrml. (Registration at IANA)

text

Text-only data including any human-readable content, source code, or textual data such as comma-separated value (CSV) formatted data. Examples include text/plain, text/csv, and text/html. (Registration at IANA)

video

Video data or files, such as MP4 movies (video/mp4). (Registration at IANA)

For text documents without a specific subtype, text/plain should be used. Similarly, for binary documents without a specific or known subtype, application/octet-stream should be used.

Multipart types

Multipart types indicate a category of document broken into pieces, often with different MIME types; they can also be used — especially in email scenarios — to represent multiple, separate files which are all part of the same transaction. They represent a composite document.

With the exception of multipart/form-data, used in the POST method of HTML Forms, and multipart/byteranges, used with 206 Partial Content to send part of a document, HTTP doesn't handle multipart documents in a special way: the message is transmitted to the browser (which will likely show a "Save As" window if it doesn't know how to display the document).

There are two multipart types:

message

A message that encapsulates other messages. This can be used, for instance, to represent an email that includes a forwarded message as part of its data, or to allow sending very large messages in chunks as if it were multiple messages. Examples include message/rfc822 (for forwarded or replied-to message quoting) and message/partial to allow breaking a large message into smaller ones automatically to be reassembled by the recipient. (Registration at IANA)

multipart

Data that is comprised of multiple components which may individually have different MIME types. Examples include multipart/form-data (for data produced using the FormData API) and multipart/byteranges (defined in RFC 7233: 5.4.1 and used with HTTP's 206 "Partial Content" response returned when the fetched data is only part of the content, such as is delivered using the Range header). (Registration at IANA)

Important MIME types for Web developers

application/octet-stream

This is the default for binary files. As it means unknown binary file, browsers usually don't execute it, or even ask if it should be executed. They treat it as if the Content-Disposition header was set to attachment, and propose a "Save As" dialog.

text/plain

This is the default for textual files. Even if it really means "unknown textual file," browsers assume they can display it.

text/css

CSS files used to style a Web page must be sent with text/css. If a server doesn't recognize the .css suffix for CSS files, it may send them with text/plain or application/octet-stream MIME types. If so, they won't be recognized as CSS by most browsers and will be ignored.

text/html

All HTML content should be served with this type. Alternative MIME types for XHTML (like application/xhtml+xml) are mostly useless nowadays.

text/javascript

Per the HTML specification, JavaScript files should always be served using the MIME type text/javascript. No other values are considered valid, and using any of those may result in scripts that do not load or run.

For historical reasons, the MIME Sniffing Standard (the definition of how browsers should interpret media types and figure out what to do with content that doesn't have a valid one) allows JavaScript to be served using any MIME type that essentially matches any of the following:

• application/javascript
• application/ecmascript
• application/x-ecmascript
• application/x-javascript
• text/javascript
• text/ecmascript
• text/javascript1.0
• text/javascript1.1
• text/javascript1.2
• text/javascript1.3
• text/javascript1.4
• text/javascript1.5
• text/jscript
• text/livescript
• text/x-ecmascript
• text/x-javascript

Some content you find may have a charset parameter at the end of the text/javascript media type, to specify the character set used to represent the code's content. This is not valid, and in most cases will result in a script not being loaded.

Image types

Files whose MIME type is image contain image data. The subtype specifies which specific image file format the data represents. Only a few image types are used commonly enough to be considered safe for use on web pages:

Audio and video types

As is the case for images, HTML doesn't mandate that web browsers support any specific file and codec types for the <audio> and <video> elements, so it's important to consider your target audience and the range of browsers (and versions of those browsers) they may be using when choosing the file type and codecs to use for media.

Our media container formats guide provides a list of the file types that are commonly supported by web browsers, including information about what their special use cases may be, any drawbacks they have, and compatibility information, along with other details.

The audio codec and video codec guides list the various codecs that web browsers often support, providing compatibility details along with technical information such as how many audio channels they support, what sort of compression is used, and what bit rates and so forth they're useful at. The codecs used by WebRTC guide expands upon this by specifically covering the codecs supported by the major web browsers, so you can choose the codecs that best cover the range of browsers you wish to support.

As for MIME types of audio or video files, they typically specify the container format (file type). The optional codecs parameter can be added to the MIME type to further specify which codecs to use and what options were used to encode the media, such as codec profile, level, or other such information.

The most commonly used MIME types used for web content are listed below. This isn't a complete list of all the types that may be available, however. See the media container formats guide for that.

multipart/form-data

The multipart/form-data type can be used when sending the values of a completed HTML Form from browser to server.

As a multipart document format, it consists of different parts, delimited by a boundary (a string starting with a double dash --). Each part is its own entity with its own HTTP headers, Content-Disposition, and Content-Type for file uploading fields.

Content-Type: multipart/form-data; boundary=aBoundaryString
(other headers associated with the multipart document as a whole)

--aBoundaryString
Content-Disposition: form-data; name="myFile"; filename="img.jpg"
Content-Type: image/jpeg

(data)
--aBoundaryString
Content-Disposition: form-data; name="myField"

(data)
--aBoundaryString
(more subparts)
--aBoundaryString--

The following <form>:

<form action="http://localhost:8000/" method="post" enctype="multipart/form-data">
  <label>Name: <input name="myTextField" value="Test"></label>
  <label><input type="checkbox" name="myCheckBox"> Check</label>
  <label>Upload file: <input type="file" name="myFile" value="test.txt"></label>
  <button>Send the file</button>
</form>

will send this message:

POST / HTTP/1.1
Host: localhost:8000
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:50.0) Gecko/20100101 Firefox/50.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Connection: keep-alive
Upgrade-Insecure-Requests: 1
Content-Type: multipart/form-data; boundary=---------------------------8721656041911415653955004498
Content-Length: 465

-----------------------------8721656041911415653955004498
Content-Disposition: form-data; name="myTextField"

Test
-----------------------------8721656041911415653955004498
Content-Disposition: form-data; name="myCheckBox"

on
-----------------------------8721656041911415653955004498
Content-Disposition: form-data; name="myFile"; filename="test.txt"
Content-Type: text/plain

Simple file.
-----------------------------8721656041911415653955004498--

multipart/byteranges

The multipart/byteranges MIME type is used to send partial responses to the browser.

When the 206 Partial Content status code is sent, this MIME type indicates that the document is composed of several parts, one for each of the requested ranges. Like other multipart types, the Content-Type uses a boundary to separate the pieces. Each piece has a Content-Type header with its actual type and a Content-Range of the range it represents.

HTTP/1.1 206 Partial Content
Accept-Ranges: bytes
Content-Type: multipart/byteranges; boundary=3d6b6a416f9b5
Content-Length: 385

--3d6b6a416f9b5
Content-Type: text/html
Content-Range: bytes 100-200/1270

eta http-equiv="Content-type" content="text/html; charset=utf-8" />
    <meta name="vieport" content
--3d6b6a416f9b5
Content-Type: text/html
Content-Range: bytes 300-400/1270

-color: #f0f0f2;
        margin: 0;
        padding: 0;
        font-family: "Open Sans", "Helvetica
--3d6b6a416f9b5--

Importance of setting the correct MIME type

Most web servers send unrecognized resources as the application/octet-stream MIME type. For security reasons, most browsers do not allow setting a custom default action for such resources, forcing the user to save it to disk to use it.

Some common incorrect server configurations:

• RAR-compressed files. In this case, the ideal would be the true type of the original files; this is often impossible as .RAR files can hold several resources of different types. In this case, configure the server to send application/x-rar-compressed.
• Audio and video. Only resources with the correct MIME Type will be played in <video> or <audio> elements. Be sure to specify the correct media type for audio and video.
• Proprietary file types. Avoid using application/octet-stream as most browsers do not allow defining a default behavior (like "Open in Word") for this generic MIME type. A specific type like application/vnd.mspowerpoint lets users open such files automatically in the presentation software of their choice.

MIME sniffing

In the absence of a MIME type, or in certain cases where browsers believe they are incorrect, browsers may perform MIME sniffing — guessing the correct MIME type by looking at the bytes of the resource.

Each browser performs MIME sniffing differently and under different circumstances. (For example, Safari will look at the file extension in the URL if the sent MIME type is unsuitable.) There are security concerns as some MIME types represent executable content. Servers can prevent MIME sniffing by sending the X-Content-Type-Options header.

Other methods of conveying document type

MIME types are not the only way to convey document type information:

• Filename suffixes are sometimes used, especially on Microsoft Windows. Not all operating systems consider these suffixes meaningful (such as Linux and MacOS), and there is no guarantee they are correct.
• Magic numbers. The syntax of different formats allows file-type inference by looking at their byte structure. For example, GIF files start with the 47 49 46 38 39 hexadecimal value (GIF89), and PNG files with 89 50 4E 47 (.PNG). Not all file types have magic numbers, so this is not 100% reliable either.

See also

• Web media technologies
• Guide to media types used on the web
• Properly configuring server MIME types