Support "image/png" MIME-type

There is a wrong MIME-type for ".png" files on internal pages.
It sets 'text/html' for them. So we have some broken images like this:

chrome://downloads/1x/incognito_marker.png

Change-Id: I61f241c87fac50d8fbb65876536c7d0ef06358d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1496979
Reviewed-by: Dan Beam <dbeam@chromium.org>
Commit-Queue: Alexander Yashkin <a-v-y@yandex-team.ru>
Cr-Commit-Position: refs/heads/master@{#637612}
2 files changed