1) I frequently tweak things in Roundcube, although not at the time of upgrade. 2) Mime Typing? Maybe, I do use a pretty extensive mime.types file c/o Apache. I notice that with 1.3.4, a mimetypes.php file appears in the config directory; who knows why? Funny, there aren't any image types specified in that mimetypes.php file at all. 3) Yes, Roundcube's changelogs don't mention anything. But they don't mention the mimetypes.php file, either.
Strange thing yesterday, was that I fired up Firefox and logged in and it worked fine. I went back to Chrome, which I normally use, and it looked fine there also. Great! I logged off and back on with Chrome. Still good. Existed Chrome, restarted, and logged back in. Still good! Reset Apache 2.4 server, and Chrome did not render the SVG images. Firefox continued to work. Ran Chrome's Developer Tools; nothing unusual observed. Nothing logged in the Apache logs, or Roudcube's logs/errors. Ran some errands, and came back to find Chrome working fine. Firefox continues to work. 15 hours later, things are still working. This is a strange problem. Roundcube's image functions are straight-up PHP, no Javascript.
I will continue to monitor.