Postfix by default restrict attachment size to approx 10MB i.e. 10240000 bytes.
You can check it using following command:
postconf | grep message_size_limit
To change attachment-size to say 50 MB, run a command like:
postconf -e message_size_limit=52428800
Note:
If you are running a mail-server with SMTP/IMAP access, you need to change postfix attachment size only. I spent half-hour debugging dovecot to increase attachment size, just to realize that above change in postfix config was all I needed!