The best way to do this would be to go with mcafee's network DLP Prevent appliance solution. When properly configured with the MWG it will send all uploads through ICAP to the prevent where it gets a) kepts in the capture database (these appliances are 2U walls of high density hard disk with a specialized file system for saving these goodies), b) logged in detail, c) evaluated against network dlp policy.
I haven't really looked at the on-box dlp, then again for most of my clients (in finance) I block dropbox for all but a couple authorized users, and network dlp keeps tabs on them with the method above.
If you'll able to filter HTTPS trafic, then you should be able to filter uploaded files - don't forget to enable composite openers, and you should have access to text, extracted from documents/archives, etc. This text later could be analyzed using built-in DLP functionality.