I've been having a play with the Drag 'n' Drop zimlet, allowing me to drag and drop files from the OS directly into a mail as an attachment or into the Briefcase (Get or contribute Zimlets, UI themes, and languages in the Zimbra Gallery - Drag and Drop Firefox XPI[BETA])
Looks to work great on Win XP, but there's an issue on Linux.
I'm running Ubuntu Linux with Gnome desktop. Drag and drop direct from the desktop into Zimbra works fine. But when I drag and drop from the Gnome file browser (which is a bit more likely than the desktop) it opens the file in the browser rather than attaching it.
From XP I can drag and drop files (including multiple files in one go) from Windows Explorer into Zimbra.
I'm happy to raise a bug, but would be useful for someone else to verify this behaviour first.