Right now, it seems xffm remembers which application go with which files, but checking for the first period. For example, I open Firestorm.LSA.avi with xine and I tell xffm to remember that. If i look in application.xml, I see <extension id=".LSA.avi" application="xine"/>. xffm seems to look for the first period and remember everything after it. I think it would probably be more desirable to have xffm looks for the last period. So that <extension id=".avi" application="xine"/> would affect Firestorm.LSA.avi also, which it does not right now.
The problem with what you suggest is with files like foo.ps.gz, foo.tar.gz and foo.gz. The first should open with gv, the second with tar -zxf and the third with gunzip. Also, if you open and remember a file foo.avi, then <extension id=".avi" application="xine"/> will be saved. This will affect all files like foo.whatever.avi, unless there is a <extension id="whatever.avi"> registered, which will take precedence.