To use an mp3 as a ring tone, open up media manager. When you have selected the mp3 you want, start playing it. Hit the menu button and you have an option to "set as phone tune". You can also copy an mp3 to the ring tones subdirectory as well. I'd suggest editing the MP3 from its full length to something about 30-40 seconds so as to save disk space if you want just a ring tone.
FYI, if you use a mp3 with a foreign character set for a name, it won't display the name properly in the file manager section but if the embedded mp3 info is in a foreign language it appears to display fine in the mp3 window when it is playing. For example I have Russian mp3 with their native Russian file names in Russian. In the file manager it gives me some weird --1820~.mp3 for the file name when it should be Земфира_Город.mp3. You can see what I wrote if you have Cyrillic encoding setup in your browser. Within the media player itself it shows the correct names in Russian as I have them encoded in the mp3 tag info.
As for attachments, you asked about "work". Did you mean to say "word"? You I have viewed Word and Excel with no issue. I am using MS Office 2003 Pro and it supports those file types with no issue. I can't speak yet for Office 2007 as I have it but haven't tried sending one to the 8100.