(Applies to: all versions of FusionBB)
If file attachments are enabled, when making a post you can click the
Manage Files link. A popup window will open allowing you to browse and select file(s) off your hard drive, then click
Add. After the file has shown up in the section below, click
Finished and the window will close so you can continue making your post.
Be sure that the file(s) you're adding are at or below the limit set by the site administrator. The size limit should be noted near the listing of allowed file extensions.