Asanka: I can't figure out who to assign this to, as it started flaking in 
https://ci.chromium.org/buildbot/chromium.win/Win10%20Tests%20x64/21973
with no clear cl that causes it
and since
https://ci.chromium.org/buildbot/chromium.win/Win10%20Tests%20x64/21984
it's been consistently failing.

I'll have a look.

btw this might be related to switching bots from Win 10 RS1 to RS2. see bug 818381. The rollout started on 3/29 and took roughly 24 hours to complete.

Actually, this also looks to have existed for a while (https://productforums.google.com/forum/#!topic/chrome/2poA-_1YwRM)

The switch probably only surfaced the change.

Also https://www.cnet.com/forums/discussions/jpeg-to-jfif-automatically-help/, from much longer ago... (the comments point at it being new Win10 behavior, at any rate).

Chrome is respecting the host OS's preference for mapping a MIME type to an extension, which in itself isn't a bug.

However, the tests shouldn't rely on local configuration. So I'm modifying the tests  to be resilient to this kind of change.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/97ae36c2bb3c9d0c29ef73db3c8117d1eb911c32

commit 97ae36c2bb3c9d0c29ef73db3c8117d1eb911c32
Author: Asanka Herath <asanka@chromium.org>
Date: Tue Apr 03 14:47:07 2018

Clean up net/base/filename_util_unittests

Fixups include:
* Additional documentation on what is being tested.
* Removal of duplicate test cases.
* Removal of test parameters that were not being exercised.

Bug:  828038 
Change-Id: I2ec965c72c0872b402b92914863e4afa26de6ebc
Reviewed-on: https://chromium-review.googlesource.com/990957
Commit-Queue: Asanka Herath <asanka@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547703}
[modify] https://crrev.com/97ae36c2bb3c9d0c29ef73db3c8117d1eb911c32/net/base/filename_util_unittest.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/27b7af9fb368ab062cb27a690e3c155195be0f1a

commit 27b7af9fb368ab062cb27a690e3c155195be0f1a
Author: Asanka Herath <asanka@chromium.org>
Date: Tue Apr 03 14:52:19 2018

Make net/base/filename_util_unittests not depend on local configuration.

Some of the tests run by filename_util_unittests invoke
GenerateFilename() which in turn consults the host platform's MIME type
to file extension map when determining a filename. This causes tests to
fail on systems where the file extension for image/jpeg was something
other than .jpg.

This CL removes the MIME type dependence from tests that shouldn't be
exercising it. It also changes the MIME type from image/jpeg to
application/x-chrome-extension for the few remaining tests that do. The
latter shouldn't be set on any platform. In addition, a new tests
correctly flags the underlying cause if tests were to be run on such a
system.

Bug:  828038 
Change-Id: I7e2ea4c6147c0a8de72e4d1b5dbf46ad286a4599
Reviewed-on: https://chromium-review.googlesource.com/990998
Commit-Queue: Asanka Herath <asanka@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547706}
[modify] https://crrev.com/27b7af9fb368ab062cb27a690e3c155195be0f1a/net/base/filename_util_unittest.cc

Files getting renamed to .jfif is not super great. Chrome is reading the value of the extension property from HKCR\MIME\Database\"Content Type"\"image/jpeg".

One possible workaround for the general usability issue is to check if the local system associated the well known extension with the target MIME type, and if so, just use the well known extension. This would be a) hacky, b) probably goes too far to avoid respecting local configuration.

For this issue, let's see if the fix sticks.

 Issue 828136  has been merged into this issue.