Automatic Save Folder will not only let you filter according to filename, but also by domain. That way you can say that every file from, for example, Gmail gets placed in an attachment directory. Or you can go a little further and say that this only applies to ZIP files from Gmail.
Setting it all up might be a little time consuming if you do a thorough job, but the end result can be a very organized folder structure for your downloads. I’m kinda an organization freak so this is right up my alley.
Get Automatic Save Folder for Firefox [via Lifehacker]