I learn something new on every project.
For a recent requirement I had to lock down a SharePoint site for external users to upload files.
Did you know that SharePoint Online supports uploads of up to 10GB?
Yes. You probably did know that.
Did you know SharePoint Online allows users to drag and drop files into libraries?
Once again, probably yes you did.
Did you know that the same JavaScript library is responsible for both drag and drop operations and large file uploads?
No. No, I did not know that. ‘dragdrop.js’ Who knew?
Did you know that without the ‘Use Remote Interfaces’ permission, a user cannot drag/drop or upload files larger than 100MB?
Nope. Didn’t know that either.
Did you know that due to a bug/behaviour-by-design in SharePoint Online you need the ‘Use Remote Interfaces’ permission on the Site itself, not just the library?
Oh, crap. That’s a limitation.

Use Remote Interfaces Permission
‘Use Remote Interfaces’ – Use SOAP, Web DAV, the Client Object Model or SharePoint Designer interfaces to access the Web site.
This gives way too much privilege to users for my use case. For instance, this opens up the ability to open the Share dialog, which in this instance we do not want to show people.
The solution? Turn off ‘Use Remote Interfaces’ for the whole Site and live with uploads limited to 100MB and no drag and drop.

SharePoint Online Large File Upload and Drag/Drop Error
If a user tries to upload a big file or drag one in, they are eventually presented with the error: ‘Error with request to the server: Access denied. You do not have permission to perform this action or access this resource’.
At least security wins out over convenience.