.jpeg is a common extension for JPEG files. having to rename them to .jpg for use in this forum (since the forum software currently only allows .jpg, not .jpeg) in annoying.
.docx and .rar are also missing.
If you want to share documents with the internet, see Google Docs.
Seriously? 
Uhm. Yes?
It's perfect for sharing and collaborating on documents on the internet.
If you want to share finished documents, use pdf.
done
For archive purposes, today supported:
doc, gif, jpg, jpeg, pdf, png, txt, zip, tar, c, h, hex, bas, xls, odt, asm, wav, aiff, wma, mp3, flac, asc, ods, xlsx, py
done
Can I suggest that if it is not already done you block or at least require moderation approval for posters with with low post counts (sub 1 - sub 5?) adding attachments if you are going to allow this expanded range of files. Much as everyone should use common sense with downloads not everyone has that
Also odf and docx would be the other two to complete the set.
If you want to share documents with the internet, see Google Docs.
Seriously?
Good reasons to use Google docs occasionally for forum sharing due to the file size limit. I did this last week for a fat PDF for one of the members.