I had a similar problem and fixed it today.
When I tried to upload an attachment, nothing would happen. I wouldn't even get an error message.
I was not able to upload e-mail attachments with Firefox or Chrome, and Internet Explorer wouldn't even open any websites.
The problem may be because you accidentally changed the internet settings for the whole computer somehow and did not realize it. This could have been done by several ways, including but not limited to, fooling around with the Internet Explorer settings.
I use Firefox regularly but I fixed this problem by opening Internet Explorer and doing these steps.
Tools -> Internet Options -> Advanced
When in the Advanced tab, did these two things
1) Click "Restore Advanced Settings"
2) Click Reset (and OK to all the message warnings)
After I did that, I was able to upload e-mail attachments on g-mail with Firefox, Chrome, and Internet Explorer.