I decided to validate this claim by trying to send an email with an attachment from Gmail to myself and I it seemed to hold up as I never received the mail but two days later Gmail sent me a "Delivery Status Notification (Delay)" saying the message could not be delivered because of the following reasons:

Technical details of temporary failure:
TEMP_FAILURE: Unspecified Error (SENT_MESSAGE): Connection timed out

Since my postfix server (behind my firewall) was not even logging a connection attempt from Gmail, I suspected it had to be something with the stateful inspection of ESTMP on the ASA. I removed stateful inspection of ESMTP from the global ploicy map:

ASA(config)# policy-map global_policy
ASA(config-pmap)# class inspection_default
ASA(config-pmap)# no inspect esmtp

and walla I was now able to recieve email with attachments from Gmail again.