There is a bug about saving files with unexpected type about deviceStorage, please see this first:
https://bugzilla.mozilla.org/show_bug.cgi?id=852864

I think if it's an .ogg file, the returned type from MimeMapper.guessTypeFromExtension() should always be "video/ogg", then we should encounter an error taht the blob can not be saved. However, the apps that will trigger the open activity of music app should also use MimeMapper to guess the type(like email or sms), so for the files with type "video/ogg", they will be opened in video app because music app only accept "audio/ogg". So is it possible for you to have audio files with "video/ogg" in the current music open activity?