Don't filter attachment types

Mandrill dropped filtering on attachment mimetypes, so stop enforcing this in Djrill.
Fixes #26

djrill/mail/backends/djrill.py

@@ -233,18 +233,6 @@ class DjrillBackend(BaseEmailBackend):
         if mimetype is None:
             mimetype = DEFAULT_ATTACHMENT_MIME_TYPE
 
-        # Mandrill silently filters attachments with unsupported mimetypes.
-        # This can be confusing, so we raise an exception instead.
-        (main, sub) = mimetype.lower().split('/')
-        attachment_allowed = (
-            main == 'text' or main == 'image'
-            or (main == 'application' and sub == 'pdf'))
-        if not attachment_allowed:
-            raise NotSupportedByMandrillError(
-                "Invalid attachment mimetype '%s'. Mandrill only supports "
-                "text/*, image/*, and application/pdf attachments."
-                % mimetype)
-
         try:
             content_b64 = b64encode(content)
         except TypeError: