R1:7a93b443c3d3

Make file upload policies more consistent

Summary:
Ref T7149. Currently, global drag and drop always uses the most open visibility policy on the install. This was appropriate before the application preference was introduced, but default to the application preference now.

In particular, this supports a default value of "Administrators" in the Phacility cluster.

Also simplify/clean up some code.

Test Plan:
- Set application default policy to "Adminstrators".
- Uploaded file via drag-and-drop, saw…