Blob.type

var mimetype = blob.type

var i, fileInput, files, allowedFileTypes;

// fileInput is a HTMLInputElement: <input type="file" multiple id="myfileinput">
fileInput = document.getElementById("myfileinput");

// files is a FileList object (similar to NodeList)
files = fileInput.files;

// our application only allows GIF, PNG, and JPEG images
allowedFileTypes = ["image/png", "image/jpeg", "image/gif"];

for (i = 0; i < files.length; i++) {
  // Test if file.type is an allowed file type.
  if (allowedFileTypes.indexOf(files[i].type) > -1) {
    // file type matched is one of allowed file types. Do something here.
  }
});