You could achieve this if you have the system that is generating the document send that document directly to Mendix, possibly through an published REST endpoint. For example, if the file is a receipt generated by your billing system, you could have that system send it to Mendix with some metadata and save it without human intervention. This requires your generating system to have the ability to make API calls.
There is not an out-of-the-box to manually drag a file from a program like an e-mail client (this is where I have seen this requirement most often) directly into the Mendix drop zone. I think there are some more advanced custom solutions (like outlook add-ins) that might work for your specific use case.