Since the 4.14 update, multiformat messages are always displayed in plain text, even though the option "prefer HTML to plain text" is enabled in the settings.
Same on my system, but this fixed the problem for me:
Disable "Prefer HTML to plain text"
Close Kontact or KMail and wait a few seconds
Start Kontact or KMail again and reenbale "Prefer HTML to plain text"
Disable/Reenable without closing Kontact/KMail didn't work for me.
Could you provide me an test email ?
In my case it was every mail with plain text and HTML Content (always the plain text version was displayed).
Since I made the changes as I wrote, it works as it should (HTML Content is displayed)
I confirm that the steps in Comment 1 fix this for me too. Every multipart message was affected (for instance notification emails from bugs.kde.org)
Since I updated my openSUSE 13.1 system to KDE Applications 4.14 all messages are displayed in plain test even though I have the prefer HTML box checked. To view the HTML message I have to do it manually.
I just did what was mentioned in Comment 1 and it fixed the problem.