Fix DocumentsUI causes Gmail to crash when performing drag-n-drop

of a directory

Add defensive check to prevent extra invalid mime type to be
included in a union way of Set.addAll() API call.

Bug: 131121225
Test: manual
Test: atest DocumentsUIGoogleTests
Change-Id: Idceb7eaac5b5f102bd8932ccb2b187a07b794baf