My CBB 3.0.6 was set up with all the defaults intact.
Allowed extensions: zip gif jpg

Attaching file (.jpg file) to a post can cause
"View all posts" and "View all topics" links to hang
and the post itself not viewable (displays a blank).
Strange thing is that not all posts with attached
images cause this. Some are perfectly alright.

What causes these?

I am having:
Xoops 2.0.13 phpkaox theme.
MySQL is 4.1.12
PHP is 4.3.9
Fedora Core 2

Perhaps you should upgrade your XOOPS to 2.0.15.