When saving an image in photos it seems that there are two names. One which is inside photo (something like IMG_123.png) and one which is the filename.
Depending of how you include the image in other application (using the "share" option inside photo or using "attach file in gmail") either name can be used.

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chrome/ios_internal.git/+/51b7e98f359166515ba0cbaa4be62a6efa45018a

commit 51b7e98f359166515ba0cbaa4be62a6efa45018a
Author: gambard <gambard@google.com>
Date: Fri Oct 21 18:02:42 2016

+jasonkliu@ as you were mentioning the .png name from safari.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/16cfe1ff158f1190e1b26eb1e491409ae904a9b8

commit 16cfe1ff158f1190e1b26eb1e491409ae904a9b8
Author: gambard <gambard@chromium.org>
Date: Wed Dec 21 13:12:09 2016

Remove image name

This CL removes the image name and add a unique filename with a default PNG
extension. Before this CL the images were named with their filename. But in
order to be saved to the device, the file should have a valid extension.

Revert of the resolution of 656641

BUG= 656641 , 674555
TEST=Download images with different image type (gif, tiff, bmp, jpg, png)
and verify it is working as expected.

Review-Url: https://codereview.chromium.org/2595573002
Cr-Commit-Position: refs/heads/master@{#440080}

[modify] https://crrev.com/16cfe1ff158f1190e1b26eb1e491409ae904a9b8/ios/chrome/browser/ui/browser_view_controller.mm

This broke the download, so I am reopening it and making it available.
We should probably re-use the code used to determine the mime type to know the type of the image downloaded and append the correct extension.

Verified on chrome beta version 57.0.2987.53 on iPhone 6 plus with iOS 10.3.

Steps followed for verification:
1. Save a JPEG file, PNG, BMP or GIF file onto device(Long press on image --> Save
2. Go to Photos --> Tap on JPEG, PNG, BMP or GIF file --> Share --> Mail --> Send it to an email@email.com
3. Go to email.com --> Sign in --> Verify that JPEG, PNG, BMP, OR GIF is seen respectively when mouse is placed on the images

Looks good.