The problem is caused by Application Control incorrectly associating the URL https://www.yousendit.com/folders?phi_action=app/addFileToWorkspaceFolder with downloading files only. In fact, this URL is common to both the upload and download processes for this web site. Consequently, blocking access to this URL to prevent downloading files will adversely affect the upload process.
To fix the problem, it is necessary to allow access to this specific URL before other rules that allow or deny access to the web site. This can be done in Visual Policy Manager.