Mail Sender Adapter with Attachment

Hi all,

I've a problem in my scenario. If it reads a message from a 3rd system it doesn't detect the attachment correctly. I've seen that the content-description part, which will be used to detect the attachment name, is empty.

But when I forward the same mail to the receiver mail address, it will be correctly detected. Why?

I've tried the variants base64 or none. But the result is the same.