I'm having a really curious problem with zimbra (Collaboration Suite 6.0) webmail on Firefox 3.6.3 (on windows XP or MAC), I'm not able to attach a file greater than 1024ko. I mean once I click on "Attach", the window is indicating "attaching" but never does anything.
If the file size is less than 1024ko, the file will be successfully attached to the email.
Furthermore, if I use Chrome 5 or Opera 10.53 on the same computer I'm able to properly attach files greater than 1024ko to my emails.

My max upload size parameter is properly set on 10Mo and I'm able to send messages with 10Mo attachment files with thunderbird.

What can I do ?