I know this is an older post, but for anyone else who may find it:
The Discussion Forums Add-On has the ability to attach files:
Files and Attachments
-Attachments to posts are fully supported.
-Multiple attachments to a post can be posted and edited.
-Files are completely integrated with the concrete5 file manager, meaning that they can be administered, searched, edited and interacted with using the file manager. File permissions are also honored.
Here is a link to the Add-On:
I hope this is helpful,