MyBB Community Forums

I get this every time I try to post a thread
Please correct the following errors before continuing:
The type of file that you attached is not allowed. Please remove the attachment or choose a different type.

If the type of the file you're uploading is not registered in your MyBB configuration, the system will prohibit you from uploading it. Please review your attachment type configuration in AdminCP > Configuration > Attachment Types. See https://docs.mybb.com/1.6/Admin-CP-Config-Attachments/ for detail.

Maybe the file you attached is not supported or very big than the forum can accept. but did you read the details about attachments.