http://voice.firefallpro.com/2012/03/html5-audio-video-mime-types.html

The "body" using contains gets access to the normally non visible body parts, so you could use a filter the check for the MIME type of MP3. So you could do a filter on "Content-Type: audio/mpeg;" But that is where it gets messy. MP3 is a derivative on MPEG. So your files may be listed as audio/mpeg or audio/mp3 or a number of others. This site list a few for HTML 5 http://voice.firefallpro.com/2012/03/html5-audio-video-mime-types.html Wikipedia also has a table of audio mime type http://en.wikipedia.org/wiki/Internet_media_type Unfortunately in a world of the internet not everyone uses the standards and there are always rugged individuals determined to do it their way. see http://stackoverflow.com/questions/10688588/which-mime-type-should-i-use-for-mp3 Therefore you may receive email from some clients with the type application/octet stream and an expectation that the file name will decipher the content type (Like windows uses) and other subtle variations.

Thank you for the information. I will study, and see if I can get a clue. Robert

[https://addons.mozilla.org/en-us/thunderbird/addon/filtaquilla/ FiltaQuilla] may be useful if you find what you want to do is beyond the built-in filtering capabilities.