[Backend] Returns with code 500 upon submission of wrong URL

Backend returns with code **500 (Internal server error** when a file with wrong extension is submitted for a challenge. It should return with **400 (Bad request)**

Example: Submitting a `.py` file URL to a challenge that doesn't accept `.py` files

If you'd like to report a bug, please include the following information:
- Expected behaviour
It should return with a `400 Bad Request`

- Observed behaviour
Server gives a `500 Internal Server Error`

- Steps to reproduce the problem
1. Choose any challenge
2. Submit a file URL with `.py` (or `.cpp`, etc) extension
3. Observe the error

Upon submitting a file URL with `.py` extension

Start by reproducing the submission with an unsupported file extension, such as `.py`, and trace the backend request handling described in the issue. Done means the same invalid submission returns HTTP 400 instead of HTTP 500, with a regression test covering the behavior.

Written by the indexing model from the issue text.