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

[aniketkulkarni17]

Hi,
I would like to work on this issue. Can you please assign this issue to me? Also, would like to know which filetypes should be accepted so that I can handle the errors for other file types

[mathemancer]

This is still blocked pending product approval. It's not clear when/if we're going to support other filetype uploading in Mathesar.

[mathemancer]

Okay, I've had a back channel conversation and done some thinking. Go ahead and work on this if you'd like, @aniketkulkarni17 . As mentioned in #3992, you will be assigned to this issue if you raise a reasonable PR to attempt to solve it. Thank you!