Uploadable filetypes can't be imported

[mathemancer]

Description

We currently allow uploading and storing a number of filetypes that we don't actually support for import:

• json
• Excel
• etc. (probably)

Expected behavior

We should throw a reasonable error whenever a user attempts uploading a file type we can't import.

To Reproduce

Try to import an Excel file in Mathesar.

Environment

• OS: (eg. macOS 10.14.6; Fedora 32)
• Browser: (eg. Safari; Firefox)
• Browser Version: (eg. 13; 73)
• Other info:

Additional context