Add support to mime types in Files app UI.

ui/file_manager/gallery/js/gallery_util.js

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 var GalleryUtil = {};
 
 /**
  * Obtains the entry set from the entries passed from onLaunched events.
  * If an single entry is specified, the function returns all entries in the same
  * directory. Otherwise the function returns the passed entries.
@@ skipping 27 matching lines @@
         return originalEntries[0].toURL() === entry.toURL() ||
             entry.name[0] !== '.';
       });
     });
   } else {
     entriesPromise = Promise.resolve(originalEntries);
   }
 
   return entriesPromise.then(function(entries) {
     return entries.filter(function(entry) {
+      // Currently the gallery doesn't support mime types, so checking by
+      // file extensions is enough.
       return FileType.isImage(entry) || FileType.isRaw(entry);
     }).sort(function(a, b) {
       return a.name.localeCompare(b.name);
     });
   });
 };
 
 /**
  * Returns true if entry is on MTP volume.
  * @param {!Entry} entry An entry.
  * @param {!VolumeManagerWrapper} volumeManager Volume manager.
  * @return True if entry is on MTP volume.
  */
 GalleryUtil.isOnMTPVolume = function(entry, volumeManager) {
   var volumeInfo = volumeManager.getVolumeInfo(entry);
   return volumeInfo &&
       volumeInfo.volumeType === VolumeManagerCommon.VolumeType.MTP;
 }