Fix support for MP4 files that are actually M4V files (#11210)

Resolve #11187

app/models/concerns/attachmentable.rb

@@ -60,7 +60,9 @@ module Attachmentable
   end
 
   def calculated_content_type(attachment)
-    Paperclip.run('file', '-b --mime :file', file: attachment.queued_for_write[:original].path).split(/[:;\s]+/).first.chomp
+    content_type = Paperclip.run('file', '-b --mime :file', file: attachment.queued_for_write[:original].path).split(/[:;\s]+/).first.chomp
+    content_type = 'video/mp4' if content_type == 'video/x-m4v'
+    content_type
   rescue Terrapin::CommandLineError
     ''
   end