Which kinds of attachments (extensions of them) are working, and which are not?
Sorry - I can't find much detail on why this is happening. I spent 2 hours Googling last night for possible solutions and then compared your PHP configuration to mine.
I've got to go to college in a few minutes and when I get home I'll try to organise some test scripts so we can work this out.
I tested .txt and .jpgs.. but uploading a jpg for an avatar doesn't work.
So I suppose I can upload all extensions.
OK, thanks!
So GIF and PNG file types work when you upload them as avatars?
No, I can't upload any image for the avatars.
But what I said was ment for uploading attachments.
I just found something weird! Maybe this can help you. I had a standard avatar for newly registered users... (thanks to zaher1988;-))) But even that isn't showing up anymore. 
Sorry about the lack of response here, destroyer.
Text file attachments =worked fine?
Image attachments to posts = ?
ZIP file attachments to posts = ?
No problem, Chris.
Text file attachments and image attachments... I just can't upload them anymore! (I know, a while ago, I was able to upload them, when I disabled Gzip.. but it doesn't work anymore, weird)
And, I'm not allowed to add zip-archives.
I made an account there the other day (surfitest). Are you able to give that account Administrator access (All I need are the permissions to access the board settings).
Chris
New theory.
I have a feeling it has something to do with the "compress attachments" plugin you have installed.
Are you able to upload fresh unmodified copies of attachment.php and inc/functions_upload.php?
I disabled the plugin but to be sure it is gone I need those two original files uploaded.
Chris