"ERROR: Unsupported format or corrupted file" MP3 Diags says: "No MPEG audio stream found"

[ohrenkino]

Did you transform them with some kind of online source?
So that they are actually mp4 files with the wrong extension?
Like here:

MP3 Tag Not Visible in File Explorer in Windows 10

I have edited several MP3 files using MP3TAG, however none of the edits I made are showing up in the File Explorer for Windows 10.