Text selection dragging triggers PDF drop

[pzhine]

Observed behavior
Steps to reproduce the behavior:

1. Select some text
2. Drag the selection
3. PDF drop UI appears

Expected behavior
We shouldn't show the PDF drop UI unless a file is being dropped. We can use the accepts prop of react-dnd to control this: https://react-dnd.github.io/react-dnd/examples/dustbin/multiple-targets

Screen.Recording.2020-12-30.at.8.08.45.PM.mov