Bug: Upload Accepts Non-CSV through Drag-and-Drop

[danglorioso]

Describe the bug

• You can drag-and-drop non-spreadsheet files on the Upload page and then proceed to the next tab anyway (??).

Where Is Bug/How to Recreate

Steps to reproduce the behavior:

1. Go to upload page
2. Drag in a non-spreadsheet file & input year
3. Push continue again and again

Expected behavior

Do not accept non-spreadsheet files no matter what.

Suggested fix(es):

Screenshots

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