did you tried increasing the debug level of Email Engine and see if it gives you some idea.
How to Increase Debug Level of Email Engine- BMC Remedy Various Logging.docx
I have never tried this with an attachment in Email template-
Problem comes when I map the attachment field of my custom form with attachment in the email-- I am bit doubtful here that email parser can do this.
Send the screenshot of incoming mailbox configuration.
As said by Rahul enable email log with debug mode.
Yes, the email parser do parse the attachment field. However it not working in my case.
I tired enabling the log file but in vain.
Even tried to generate stderr.log enabling them from the ARSystemMaintainance tool, this attempt also didn't generate any log file.