Added django.db.models.sql.query.Query.set_values() method to handle
query mutation.

Thanks Sergei Maertens for the report and patch.

Thanks Sean Mc Allister for providing a test.

The fallback logic which allows non-UTF-8 encoded files to be passed to
attach_file() even when a `text/*` mime type has been specified is
moved to attach(). Both functions now fall back to a content type of
`application/octet-stream`.

A side effect is that a file's content is decoded in memory instead of
opening it in text mode and reading it into a string.

Some mimetype-related logic in _create_attachment() has become
obsolete as the code moved from attach_file() to attach() already
handles this.

Follow up to fab46ce6.

Thanks Tim Graham for the review.

Thanks LaMont Jones for the report and patch.