Losing images when sending mail

We use a logo in our mail.

These are added through linked resources.
For sample code see post: 616457.

We recently update from V5.1 to V5.2.

Now when we sent the mails the images are not embedded in the files.

I include 2 files:
One is generated with V5.1. and the other with V5.2, running the same codebase.

We can see that the there are 2 differrences:
At the end the binary data for the images are added in V5.1 and in V5.2 not

At the beginning the Content-Type of V5.1 has extra info: multipart/related.
This is missing in V5.2

We also have a problem that in some cases the
stylesheet data is displayed in the mail
(see mail error picture).

What can be the cause of these issue's with the new version