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.

app/logical/media_file.rb

@@ -56,7 +56,14 @@ class MediaFile
       :swf
     when /\x1a\x45\xdf\xa3/n
       :webm
-    when /\A....ftyp(?:isom|3gp5|mp42|MSNV|avc1)/
+
+    # https://www.ftyps.com
+    # isom (common) - MP4 Base Media v1 [IS0 14496-12:2003]
+    # mp42 (common) - MP4 v2 [ISO 14496-14]
+    # 3gp5 (rare) - 3GPP Media (.3GP) Release 5
+    # avc1 (rare) - MP4 Base w/ AVC ext [ISO 14496-12:2005]
+    # M4V (rare) - Apple iTunes Video (https://en.wikipedia.org/wiki/M4V)
+    when /\A....ftyp(?:isom|3gp5|mp42|avc1|M4V)/
       :mp4
     when /\APK\x03\x04/
       :zip

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