Add python-magic for validating mimetypes

[yunimoo]

Potential solution for https://github.com/janeczku/calibre-web/issues/3044

This PR adds in mimetype checking to ensure that files being uploaded are not maliciously disguised.

Create a bash script:

# test.sh
#!/bin/bash
echo "Hello World!"

Rename the bash script: (Ensure that server allows epub)

mv test.sh test.epub

Upload the file to confirm that it is not possible as the mimetype is read differently.