Create unit tests for the file_upload view

[DjangoCRM]

Django CRM includes email marketing software.
Images are commonly used when creating newsletter templates.
The file_upload view allows users to upload image files to the CRM server.
See file massmail\views\file_upload.py

[sarkarshrayan2-max]

Hi, I would like to work on this issue. I will add tests for staff access, rendering the upload form, successful image uploads, and invalid file-extension validation.