Issue with transcoding the previously uploaded .m4a files

[thebengalboy]

Description
When you try to retranscode an old .m4a file, which was uploaded/added to media before Transcoder activated, it'll show a message like below:
"NAME" (ID 44) is MP3 file already. No need to send for transcoding

How to reproduce

• Keep the Transcoder plugin inactive
• Upload a .m4a file
• Activate the Transcoder plugin
• Now, try to retranscode the uploaded .m4a file

How it should be
As we accept retranscoding other file formats, even though uploaded before Transcoder activated, we need to do the same for .m4a file format as well.

[Pathan-Amaankhan]

I was able to replicate the issue, looking into it now.

cc: @joelabreo227 @aviral-mittal