Increase attachment size

==== Required information ====
- iRedMail version: v1.5.1 ()
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MySQL
- Linux/BSD distribution name and version:
- Related log if you're reporting an issue:
====

Hi,

I would like to know that how I can increase the attachment size on Iredmail pro. I tried options that available in different forums but that is not working.

Regards,

Arvindh S