The Postfix default attachment size is 10MB i.e. 10240000 bytes.
This command will output the currently set attachment size limit:
postconf | grep message_size_limit
To increase the limit to 50 MB, run this command:
postconf -e message_size_limit=52428800
from: https://rtcamp.com/tutorials/mail/postfix-attachment-size/
also: https://www.howtoforge.com/community/threads/increasing-email-size-limit-from-10mb.33960/