Here is what you need to do to allow more than 10mb for attachments upload.
Connect to your server via SSH
Scroll down where it says
upload_size: 20m below
params: db_default_text_search_config: "pg_catalog.english" # replace 20m with desired value upload_size: 20m
After making changes press Ctrl+O (to write) and Ctrl+X (to exit).
./launcher rebuild app
Once rebuild is complete, navigate to
forum.example.com/admin/site_settings/category/files and change the
max attachment size kb to
20480 (20mb) or your desired number. At this point everything should work perfectly, you can go on and test your upload file size.