I have opened a case to investigate this for a future release. We will update here when it is resolved. If you need further assistance, please don't hesitate to contact us via the link below.
The problem only happens when you drag a file onto the open dialog box. This makes FogBugz think you're dragging and dropping a file onto the case, which changes the form on the page. If you select the file from the dialog it should work.
I have a similar issue, when I am on Safari in Windows 7, when I attempt to click on a file in the open file dialog window, the click registers with the browser window and sends me to a different page in Fogbugz.