I must be blind because I can see bits of code for post attachments, but can't find a way to post them.

Can someone please guide me on how to post attachments? Is there some secret samurai way?

On a side note, it seems this forum uses that module and there is definitely an Attach File option on this post.

Why does mine not have it?

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)

$fileUploadField->setAllowedMaxFileSize(1000000);

to

$f_validator = new Upload_Validator();
$f_validator->setAllowedExtensions(array('pdf','png','jpg')); // set your extensions here
$f_validator->setAllowedMaxFileSize( array('*' => 1000000) );
$fileUploadField->setValidator($f_validator);