as shown above, foobar2000 v1.4 beta 11 is able to handle tags correctly regardless the extension name, but mp3tag can only read/write tags for opus files that end with .opus
While The recommended filename extension for Ogg Opus files is .opus. according to https://wiki.xiph.org/OggOpus, changing file extension name to .ogg is the only way to make Android mediaserver pick up these files. In addition, Android is able to play opus files with .ogg extension and read their tags w/o any problem since Marshmallow, according to https://android.stackexchange.com/questions/37970/what-can-play-ogg-opus-on-android. I'm also able to comfirm this on my Nexus 6P running Android 8.1.
You would get the same behaviour, if you renamed an mp3 file to flac or an mp4 file to wav or whatever.
Right now MP3tag does not do a lot of guessing what kind of file you have - because the absence of tags at a certain position or in a certain format could mean that there are not tags at all and MP3tag has to add them.
AFAIK MP3tag does not analyze the audio data (as perhaps the players do).
This is the current situation.
You're correct and this is exactly why it's not the same as renaming mp3 to flac.
Mp3tag doesn't support codec detection inside Ogg containers currently and assumes Vorbis by default. It's on my list, but for the time being, Opus needs to come with the *.opus file extension to be usable inside Mp3tag.
I think that you can use an action to rename the extension, using %_filename_ext% as field.
So you can rename files so that they reflect the proper file type in WIndows and add further information for the Android programs.
