Disclaimer: I love drag & drop, I think it makes UX sense. This being said, my current position is that it is generally not worth the complexity and reliably issues to implement in Access, but I could be wrong obviously.
I researched and developed drag & drop from Win Explorer a few months ago: in the end I canned the drag & drop part because it was a whole lot of code (i.e., all sorts of API calls and over a hundred of lines of code) to manage a whole lot of exceptions. And that was just Win Explorer; users asked for drag & drop Outlook file attachments, from Gmail attachments, etc. It was clearly a pandora's box and huge complexity. Perhaps I set it up wrong but research I did confirmed this was a lot of code. Even using the low-code "Hyperlink" field type didn't work reliably in a continuous form (i.e., after a few drag & drops the field would freeze and become completely unresponsive until the db is restarted).
In the end I showed the client the simpler alternative where they press an "Insert Attachment" button that brings up the FileDialog for about a dozen lines of code. Turns out they are fine with it and there's the classic design lesson: clients tell you what they think they want, it's your job to tell them what they need. When clients say "we want drag & drop" what they actually mean is a feature to associate files with database records, designing the exact mechanics of a cost effective UX/UI is generally not clients' expertise.
I don't want to discourage you, drag & drop is great in theory, but in practice I've come to the conclusion that it's generally not worth the costs in complexity to implement in Access given that there are much simpler alternatives that users also recognize.
Hope this helps
