If you are using DropBox through a web browser to upload/download, then it's the same as scanning any other web site.
Turn on SSL scanning, and let the users access the category or site.
If you are using the DropBox app , there is no possible way to scan the content. DropBox app does not permit any man-in-the-middle interception. You must either whitelist it like are doing or block it entirely.
eelsasser, I'm using Internet Explorer browser.
When I access other non compressed file, I can get it with no issue.
Only when trying to download the compresssed zipped file , I get:
The file you are trying to download is hosted on an untrusted website. Please contact IT Support to download the file.