Add mp4

Related to: https://github.com/dimagi/MediaUploader/pull/6

http://manage.dimagi.com/default.asp?151390#844016

If we want to upload a non default extension we can now do so without having to go into the advanced path settings
This is here so that the save button is activated to alert the user to save the new filepath.

handleChange is usually called after keyup, but there aren't any keyups here, so doing it manually