Is there a way to upload a file within an admin action in an extension only app? As far as I can tell, the admin extension UI is missing an sort of file picker or drop zone component (like the Polaris Drop Zone).
If there’s no prebuilt component, is it possible to create one with React?
No, the only components that are available are what are documented.
It’s by design for both security and a cohesive UX for merchants.
But the good news is that the Drop Zone component is a common one that has already been built for other parts of the platform like Customer Accounts. I wouldn’t be surprised if that is on it’s way to the Admin Blocks.