Go to the 'User CP' tab above. Click the attachments link (the bottom one on the left). Delete some of your olded or no longer needed attachments (make sure you have backups in case this doesn't work). Try and upload your files again....
Its probably just that you're post count isn't high enough then. The more you post the more space you get to host attachments.
I'd recommend uploading to a third party file hosting site (google 'free file hosting') and just posting the link.
The other option is to send a PM to admin (names in red on the active users panel) to see if they'll up the amount you're allowed to store, but with the recent server problems they probably won't be willing to, which is understandable.