Image in webmail signature broken and added as attachment

[echodaniel]

Impacted versions
Modoboa: 1.17
Installer used: Yes
Webserver: Nginx

When adding an image in a signature, it shows the image in Compose just fine, but upon receipt, the image is broken and instead added as an attachment.

Steps to reproduce
In the web console, go to Preferences>Webmail>Composing Messages>Signature text
Add an image by selecting Image>Upload>Choose File>Send it to the Server>OK
The image shows in the Compose field in the bottom of the signature (see below)

Current behavior
The signature image comes through broken and instead added as an attachment.

Expected behavior
The image is inlaid within the signature upon receipt as it is in the Compose dialogue.