We don't reject obviously bad files

[calpaterson]

Description

Occasionally users are inventive and try to upload file formats that we don't support

Steps to reproduce

1. http://localhost:6001/new-table/upload-file
2. Upload sqlite database file

Expected result

Some kind of 400

Actual result

500

Additional details

Nice to tell them what they've done wrong and also save me the time from investigating

CSVBASE-2F