A partial solution is to filter on category "Personal Network Storage" and command name does not equal "POST". While this prevents uploads, it also prevents authentication, if required. Adding an exception list will allow to define exceptions.
My rule goes something like this:
URL.Categories <Default> contains Personal Network Storage AND (
Command.Name does not equal "POST" OR
URL matches in list Allow Network Storage Download POST Exceptions)
The action is "Stop Rule Set" to allow the normal AV and other processing to occur further on down the policy.
Thanks for the info. I was looking at the wrong category (P2P/ File Sharing). After i posted here, I played around with the rule for Webmail that i found in the McAfee KB (McAfee KnowledgeBase - Web Gateway - How to block Web Mail attachment uploads and downloads by media type), and added extra consideration the Personal Network storage with the same files types for blocking. It appears to let me sign in to dropbox and see items but not upload.
Not sure if this is the best approach or if it leaves options where users can go around the rule, but from the little test i was able to perform (dropbox and onedrive) neither allowed me to upload files to it. Posting my results in case it helps others with similar issue.
Here is a Rule Set that pretty much does what msiemens explained above. It has the exception list needed. It also has an additional rule that blocks the link to the business solution. And it implements coaching for these sites, to help educate users. The sites covered are Dropbox and WeTransfer.