Not able to include Content-Type: text/calendar in multipart/alternative

Checklist

• I have used the search function to see if someone else has already submitted the same bug report.
• I will describe the problem with as much detail as possible.

App version

5.600

Where did you get the app from?

F-Droid

Android version

8 (Lineage OS)

Device model

Xiaomi Mi5 Plus

Steps to reproduce

As discussed at https://gitlab.com/techbeeat1/jtx/-/issues/277, an Android application sends Intends and creates an email in K9, which has this structure:

Content-Type: multipart/mixed
  Content-Type: multipart/alternative
    Content-Type: text/plain; charset=utf-8
    Content-Type: text/html; charset=utf-8
  Content-Type: text/calendar

The structure shall be instead:

Content-Type: multipart/alternative
  Content-Type: text/plain; charset=utf-8
  Content-Type: text/html; charset=utf-8
  Content-Type: text/calendar

The author of the application says he uses the Android API correctly, and it is the Email application, which does not create the correct MIME structure.

Expected behavior

Not applicable.

Actual behavior

Not applicable.

Logs

Not applicable.

Start by reproducing the Android intent described in this issue and inspect how the email application's MIME structure is assembled. Compare the generated multipart/mixed and multipart/alternative nesting with the requested structure; done means text/calendar is included alongside text/plain and text/html in multipart/alternative.