As per the discussion in the email thread , content filtering is
modified. Mailman rejects emails with image and application attachments.
Gmail or any other extension may show inline preview for an URL.
The sender can upload the attachment to file sharing service and share the
URL in the email.
Thank you, Satya and others for the suggestion.
Also, the sender doesn't receive a notification that email is rejected for
so and so reason. Is this how mailman handles content filtering?