jpeg is already handled as metadata. Your file just probably don't use the expected mapping
Well i played around and it seems like JPEG's are displayed when several other ID3 Tags are filled. I tried with purchased M4A files:
- audio track with JPEG and ID3 tags: cover displayed
- i removed the ID3 tags, cover stays: cover is not displayed
- added just ARTIST & TITLE, cover is there: cover is not displayed
VLC also seems to cache files info?! i removed the cover and put the file again into the app after deleting it and the cover is still shown!
VLC on Windows is showing the JPEG cover, like the Windows file browser. So it's not an issue with the program writing the Tags / Cover info.
I also have (bought) files where cover is not displayed and some where it is displayed.
Please check the issue by yourself.