Steps to reproduce:
1. In Safari's Develop menu, select User Agent > Firefox 4.0.1 - Mac (this workaround is necessary due to <https://bugs.webkit.org/show_bug.cgi?id=79443>).
2. Navigate to the URL (sign in to iCloud if necessary).
3. Select the Numbers tab and drag a Numbers document from the Finder into the browser window.
A progress window appears as the Numbers document is uploaded to iCloud, then it is added to the view of documents in the cloud.
The progress window appears but makes no progress. The upload never completes.
Caused by <http://trac.webkit.org/r109022>
*** This bug has been marked as a duplicate of bug 80203 ***