Tests: Mailgun has changed inline attachment reporting in some events #172
Description
Sometime between 2019-12-02 and 2019-12-05, Mailgun changed their event reporting around inline attachments. Previously, inline attachments were not included in the event's message.attachments array. Now, inline attachments are included in that array in the Accepted event. Other events (like Delivered or Opened) still include only true attachments in message.attachments.
Anymail's Mailgun live integration tests verify the attachments reported in the Accepted event, and so have been failing since this change. This only affects the test (and Mailgun's event reporting); the image is still correctly handled as an inline attachment in the sent message.