I can't upload attachments bigger than 20M on my Deskpro installation built using automated scripts.
As well as increasing the size of the max attachment in PHP (set within php.ini), you also need to update your webserver to accept larger file attachments. In this scenario, where Deskpro has been deployment using automated scripts, the webserver is nginx. You will need to update the nginx configuration file and raise the value of the following setting:
It is set at 20M so you can raise it to X.