uploads: allow uploading .m4v format videos.

Fix not being able to upload .m4v format videos as reported here:

* https://danbooru.donmai.us/forum_posts/205248
* https://github.com/danbooru/danbooru/issues/3615#issuecomment-1030950924

From https://en.wikipedia.org/wiki/M4V:

  The M4V file format is a video container format developed by Apple and
  is very similar to the MP4 format. The primary difference is that M4V
  files may optionally be protected by DRM copy protection.

This could be a problem if it allows uploading videos that are
unplayable because of DRM.

test/functional/uploads_controller_test.rb

@@ -261,6 +261,7 @@ class UploadsControllerTest < ActionDispatch::IntegrationTest
         should_upload_successfully("test/files/test-animated-86x52.gif")
         should_upload_successfully("test/files/test-300x300.mp4")
         should_upload_successfully("test/files/test-512x512.webm")
+        should_upload_successfully("test/files/test-audio.m4v")
         # should_upload_successfully("test/files/compressed.swf")
       end
 

test/unit/media_file_test.rb

@@ -93,6 +93,10 @@ class MediaFileTest < ActiveSupport::TestCase
       assert_equal(:mp4, MediaFile.open("test/files/test-300x300.mp4").file_ext)
     end
 
+    should "determine the correct extension for a m4v file" do
+      assert_equal(:mp4, MediaFile.open("test/files/test-audio.m4v").file_ext)
+    end
+
     should "determine the correct extension for a ugoira file" do
       assert_equal(:zip, MediaFile.open("test/files/ugoira.zip").file_ext)
     end