Viewer fails to save a JPEG with non-JPEG filename extension

== Steps to reproduce ==
1. Download https://upload.wikimedia.org/wikipedia/commons/thumb/e/e1/Car_crash_1.jpg/193px-Car_crash_1.jpg?download
2. Open the image in Shotwell Viewer: $ shotwell 193px-Car_crash_1.jpg
3. Select File > Save As...
4. Select OK (without changing Format from "Current", or any other parameters)
5. Enter "193px-Car_crash_1.png" (without quotes) as the filename, select OK

== What happens ==
The viewer now displays a black screen, with the text "Photo source file missing:" followed by the (.png-ending) image path entered in the dialog. The file has not been saved.

== What I expect to happen ==
For the (now misnamed) .png-ending file to have been saved, and be opened in the Viewer.

== Other notes ==
* If a file with the .png ending already exists, and I try overwriting it with the JPEG, the black error screen does not appear, and the viewer appears as if it had saved the file. But the file hasn't actually been saved; if 193px-Car_crash_1.png was an empty file before seemingly overwriting it, it remains empty.
* I'm (intentionally) not doing an actual Format change in the first Save As dialog here. If I do select PNG as the format, then saving the file does work as expected (using any filename extension).
* I used .png just as an example here. Any variant of /jpe?g/i as the extension seems to work as expected, whereas anything else (.gif, .foo etc.) results in the same "file missing" error as above.
* If the original file is a PNG file, saving it with .jpg (or any other extension for that matter) seems to work as expected.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: shotwell 0.22.0+git20160108.r1.f2fb1f7-0ubuntu1.1
ProcVersionSignature: Ubuntu 4.10.0-32.36~16.04.1-generic 4.10.17
Uname: Linux 4.10.0-32-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.10
Architecture: amd64
CurrentDesktop: Unity
Date: Mon Aug 14 17:22:25 2017
InstallationDate: Installed on 2016-10-13 (305 days ago)
InstallationMedia: Ubuntu-Server 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
SourcePackage: shotwell
UpgradeStatus: No upgrade log present (probably fresh install)