Could you show us an exact copy of the string that you use in
File>Options>Tags
that lists the file types to be read by MP3tag?
Mine looks like this and it leads to MP3tag to read webm-files:
*.aac; *.ape; *.apl; *.asf; *.flac; *.flc; *.mp+; *.mp1; *.mp2; *.mp3; *.mp4; *.m4a; *.m4b; *.m4v; *.mpc; *.ogg; *.oga; *.ofr; *.ofs; *.spx; *.tak; *.tta; *.wma; *.wv; *.tmp; *.001; *.010; *.opus; *.dsf; *.mpg; *.wav; *.aif; *.aifc; *.aiff; *.mka; *.mkv; *.sfk; *.webm
Oh yes: and how do you open these files? With drag&drop or with the functions to open a folder?