Cannot download attachment for some multipart/mixed messages with base64-encoded jpg images about roundcubemail

[pabzm]

The relevant difference is that the broken version's attachment mime-structure doesn't have a Content-Disposition-header. After adding that the message gets shown as expected.

We'll have to check if/how to fix this.

[pabzm]

Related: #7117, #5905

[alecpl]

Related to #5051. At least in this specific case we can more easily detect that the message has no html part so any images should be treated as non-inline.

[pabzm]

Currently the image in the "Not OK" email is considered an inline image by Roundcube. I fiddled with some code in program/actions/mail/show.php to make it display the image, but maybe a more general approach to handle "dangling" images would be better.

Also I figured that some code probably relates to concrete edge cases, like is_attachment() – because for a general check this is too specific. Changing that code without tests checking these edge cases is too dangerous for my taste.

@alecpl Do you have some kind of test corpus of emails to check against? I started to extend TestCases in order to check the correct rendering of emails (will open a PR later).

[alecpl]

I agree that fixing this kind of issues properly might involve quite a big refactoring. There's no easy bugs in this tracker ;).