Sep 17 2018
Doesn't seem to be completely fixed: see
Aug 27 2018
It appears that "High" has been redefined to "currently working on." So then I must ask, how is one supposed to indicate that a project should be higher priority if you've redefined "High" to mean "currently working on"? I don't see a way to vote on a bug.
Jul 14 2018
Possibly relevant additional information: GIFs scale with transparency. So it must be something that's different about the PNG scaling pipeline.
Jul 8 2018
Confirmed that it's not just PNG8, but also PNG32, the more common format. So all PNGs seem to be affected.
Dec 9 2014
Okay, I tracked down the issue. It seems that an installation of Inkscape set the mimetype of SVG files to "application/svg" in the registry. Changing it to "image/svg+xml" fixed the problem.