from http://bugs.musicbrainz.org/ticket/5875 and assuming still valid
Testing some taggerscript changes, I accidentally ended up saving a few files as ".mp3", ".mp3 (2)", etc. No problem - I fix the taggerscript, reload the albums, then resave. The results weren't what you'd expect. (I've since duplicated this with ".mp3"-named files "freshly" loaded into Picard.)
For track title "Foo", you'd expect to get some variation (depending on the naming string) of "1. Foo.mp3". Instead, ".mp3" saves as "1. Foo".
It gets even stranger if you save that newly no-extension file again. (With or without reloading the album first.) Picard now assumes that "Foo" itself is the extension, so now it saves the file as "1. Foo.foo". (This is a related bug; I'll ticket it separately. Just describing it here as it's a 2ndary permutation of the main bug).