Setting the mime-type for files stored in the private directory
Editing includes/files.mimetypes.inc is a core hack, which you should strive to avoid.

Instead, use hook_file_mimetype_mapping_alter():

function mymodule_file_mimetype_mapping_alter(&amp;$mapping) {
  // Add new MIME type 'video/webm'.
  $mapping['mimetypes']['webm'] = 'video/webm';
  // Add new extension '.webm' and map it to the 'video/webm' MIME type.
  $mapping['extensions']['webm'] = 'webm';
}