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MIME Types

Learn how to add, change, and remove a file extension to a MIME type on Pantheon.

Pantheon promotes and respects internet standards, and for MIME types, we treat the IANA Media Type List as the canonical source of what headers to serve for what file extensions. This article describes what to do if you have a file type that isn't handled by IANA, or require an alternative.

Add, Change, or Remove a File Extension to a MIME Type

We will consider:

  • Adding anything not present on Pantheon with the recommendation of IANA list
  • Adding anything not on the IANA list as application/octet-stream
  • Changing any existing suffix that does not match the IANA MIME types
  • Adding a widely recognized MIME type to a suffix that does not exist in the IANA list (on a case-by-case basis)

We will not consider:

  • Adding anything that conflicts with the IANA list
  • Removing existing MIME types that are in IANA
  • Changing our default MIME type

Canonical Reference

See the Internet Assigned Numbers Authority Media Type List.


Depending on the file type, you can write a small PHP wrapper to read the file, set the MIME type header, and send back the file with any MIME type you want.

The example below outputs a given file with a hello/world MIME type and includes browser caching headers (default of 3600 seconds) to reduce load on the server:

 * Serves a file with a custom MIME type header.
 * @param string $file - Absolute path to a file.
 * @param string $mimeType - Mime Type to serve with file. (default='text/plain')
 * @param int $cacheTime - Cache control time. (default=3600)

function serveFileCustomMIMEType($file, $mimeType = 'text/plain', $cacheTime = 3600) {
  if (file_exists($file)) {

    $fileName = sprintf('"%s"', addcslashes(basename($file), '"\\'));
    $modified = gmdate ('D, d M Y H:i:s', filemtime($file)) . ' GMT';
    $size = filesize($file);

    header('Content-Description: File Transfer');
    header('Content-Type: ' . $mimeType);
    header('Content-Disposition: attachment; filename=' . $fileName);
    header('Content-Transfer-Encoding: binary');
    header('Cache-Control: public, max-age=' . $cacheTime);
    header('Last-Modified: ' . $modified);
    header('Content-Length: ' .$size);


  if (!file_exists($file)) {
    return "File not found";

serveFileCustomMIMEType("", "hello/world");