Disable mime-type guessing in s3cmd in the task runner

Summary

Currently when running backup tasks we see the following warning:

WARNING: Module python-magic is not available. Guessing MIME types based on file extensions

And then the script continues successfully.

It's just a warning, as the script will fall back to a default binary/octet-stream mime type, and we really don't need anything else for our backup files. (mime-type informs the content type that the object will send back when accessing it through a browser from object storage. So it's ideal to set for images and videos that you want to embed in a website)

As we really don't need anything other than the default here, we should pass --no-mime-magic to the s3cmd when we call it.