I ran into this problem today. I get weird messages in both Firefox and Internet Explorer when I try to send a message in Gmail if that message has a large attachment.
Firefox: After a few minutes, I get a message saying that Gmail is wanting to download a file and asking if I want to open it or save it to disk. Type application/octet-stream.
Internet Explorer: Reports that there was a problem with the (Gmail) server.
To get the email with the large attachment to send, I have to disable protection levels in ADVANCED VIEW - ANTIVIRUS - CUSTOM LEVEL. In there, I have to deselect "Scan http traffic" and "Scan Yahoo Messenger Traffic". Yes, both. Only deselcting one will not fix the problem.
That's a lot of protection to defeat to get Gmail working.
Anyone know if this is a known problem and/or a fix is in the works, or is this going to be like the Microsoft Outlook Express issue in BitDefender 2008 that caused me to be given 2009 by the the help desk since there was no fix for that. 
Brian
Edited after further testing to determine the two settings in BitDefender that affect Gmail as described.