Why do email attachments disappear?

Question

ruby

Foreword - Letters sent by Redmine are notifications of changes in tickets.
The problem is that in some clients (both in webmords and desktop applications) not all parts of the letter are visible.
For example, in letters there are two attachments and any text. The first attachment is a picture, 11 kb, jpeg. Second attachment - rubigem, 38 kb.
The gmail webmord displays all three parts (text and both attachments) normally.
Yandex webmord displays only a picture (there is no text or rubigem).
Mail.app only displays the picture.
If you look at the source of the letter in Gmail and Yandex, then the letters will be identical (except for the recipient's address).
If you forward a letter from gmail to Yandex, the letter is displayed normally.
If you send a letter from Yandex to gmail, the letter loses its rubigem and text.
Here is a link to the original letter to Yandex mail.
Thanks for answers.

Answer 1

If I understand correctly, the jamb in the crooked compilation of multipart. You have only one
Content-Type in your letter: multipart/alternative;
What tells us that all variations of parts are alternatives (i.e. replacements) to each other. And this is clearly not what you wanted. In the letter, you must have a construction like:
Content-Type: multipart/mixed (mixed content)

Content-Type: image/jpeg

Content-Type: application/octet-stream