Fix failing CI test (broken in 2daac47d or e693f45e): make test_multipart_with_template_path_with_dots only test path with dots, and stop failing on unrelated mimetype assertion. The image/jpeg multipart content type is already tested in test_explicitly_multipart_messages.

Signed-off-by: Yehuda Katz <wycats@yehuda-katzs-macbookpro41.local>