Please refer to below admin guide, FML Block and Safe list cannot be used to bypass the content scanning which including the attachment filters.
We would suggest you to follow the below steps to disable the attachment filter just for one sender.
1. Create a new Content profile and disable the attachment filter.
2. Create a new Inbound recipient policy just for one sender that you wish to disable the attachment filter.
3. Apply the new content profile that you created on Step 1.
4. Make sure you put this new rule on top of the existing inbound recipient policies because the Fortimail will check the policies from top to bottom.
5. Since the new policy only configured for one sender so only one sender will match to the new policy and the other senders will use the other existing policies.