CMS Made Simple Forums

I am using Newsletter Made Simple Version 2.2.

When I add an image field to my template via {nms_image name='block name'}. I can select an image which is attached to the mail.

But the Content-Type is set to 'application/octet-stream;'. Thunderbird does not read it correctly, since it only displays 'image/jpeg;'.

What NMS creates:

--b1_89342036fbcc8ddeb0c6788b24fde480
Content-Type: application/octet-stream; name="test.jpg"
Content-Transfer-Encoding: base64
Content-ID:
Content-Disposition: inline; filename="test.jpg"

How it should be:

--b1_89342036fbcc8ddeb0c6788b24fde480
Content-Type: image/jpeg; name="test.jpg"
Content-Transfer-Encoding: base64
Content-ID:
Content-Disposition: inline; filename="test.jpg"

How can I teach NMS to set the right content type?