Apologies for opening an old thread but I was having similar problem.
I found it in the admin area under the topic > Access > Can users attach files?
When changing this however it raised an error
[User Notice] Upload::setAllowedMaxFileSize() is deprecated. Please use Upload_Validator::setAllowedMaxFileSize() instead
To fix change this line in Forum.php (appears twice)
$f_validator = new Upload_Validator();
$f_validator->setAllowedExtensions(array('pdf','png','jpg')); // set your extensions here
$f_validator->setAllowedMaxFileSize( array('*' => 1000000) );