First check your Global File settings.
To control the types of files that can be embedded within forum threads and replies, go to Administration > Applications > Forums and review the Attachments tab:
Here, you can customize:
- Maximum File Size: The largest sized file (in kilobytes) that can be uploaded and embedded within forum threads and replies.
- Allowed File Extensions: The list of allowed file extensions for uploads into forum threads and replies.
When you've adjusted these settings, click Save to commit them.