Can via TMG's HTTPS inspection & HTTP filtering prevent users attach files in their emails?

hi friends

i need my users be able to connect to web base mail servers (for example www.Gmail.com ) & read & write emails but within their email the be unable to attach files and send our company's important files to other persons.

i have TMG as network firewall. i have no exchange server. 

can via combination of  TMG's HTTPS inspection & HTTP filtering, i prevent attaching file in emails?

thanks alot