I had this problem too. (This fix might work on other operating systems but I am using OS X 10.5.8, Firefox 3.6.27 & GMail). Tried several things to no avail. Then closed Firefox, reopened, logged into Gmail then with Gmail open I used the refresh button on the Firefox toolbar. After this I was able to attach files again. Tested closing Gmail & opening again- still OK.

Probably a good idea to give the Mac/PC a restart as well.

Hope this helps at least some people.

I also had this problem since yesterday, but using Win7. I tried cleaning cache and cookies, restarting the browser and the pc, but nothing happened. With IE and Chrome the attachment worked fine. I hoped that with the yesterday's 3.6.28 updade it would be fixed, but it didn't. So, I saw the comment above about using the refresh button on FIREFOX (not Gmail's refresh) and tried, and the problem was gone.

So, if anybody has the same issue, try hitting the refresh button on firefox :)

And thank you, junkcat!