Change mimetype to support inline images in thunderbird

[philpagel]

Change the content type from multipart/mixed to multipart/related so
that thunderbird will correctly recognize inline images.

Closes #142

[awdeorio]

Looking good! I think the unit tests need a few minor tweaks to get them to pass. Also, have you had a chance to send a message with an inline attachment from this branch and verified that it looks correct on Thunderbird (and also Gmail)?

[philpagel]

Oops – sorry for missing the testcase. I fixed that now.

I have sent emails with this branch to 3 different email accounts I have using both html and markdown templates including inline images. The emails did render as expected in all MUAs tried:

• thunderbird (Linux)
• gmail web
• gmail app (Android)
• k-9 Mail (Android)
• t-online web email
• 1&1 ionos web email

[awdeorio]

Great! Thanks for the thorough check. One last thing: it looks like there are a few outdated comments that need a tweak:

$ ag mixed
mailmerge/template_message.py
90:        method will create a new `multipart/mixed` message, copy message
131:        multipart/mixed
137:        top-level `multipart/mixed` message.
189:        Attachments are added to the payload of a `multipart/mixed` message.
193:        multipart/mixed
202:        multipart/mixed

[awdeorio]

I'm tagging @seshrs who wrote the original attachments feature. The only exceptional case I can think of is a Markdown message without an inline attachment. It would have this message structure:

multipart/related
 └── multipart/alternative
     ├── text/plain (original markdown plaintext)
     └── text/html (converted markdown)

Instead of this one:

multipart/mixed
 └── multipart/alternative
     ├── text/plain (original markdown plaintext)
     └── text/html (converted markdown)

I tested this in GMail and it seems to work fine.

[codecov-commenter]

Codecov Report

Merging #144 (64681ce) into develop (1a695a7) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #144   +/-   ##
========================================
  Coverage    99.70%   99.70%           
========================================
  Files            5        5           
  Lines          344      344           
========================================
  Hits           343      343           
  Misses           1        1           

Impacted Files	Coverage Δ
mailmerge/template_message.py	100.00% <100.00%> (ø)

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[awdeorio]

LGTM!