`md5ext` property missing from serialization sometimes

[paulkaplan]

Expected Behavior

Please describe what should happen

There is a property on costumes called md5ext which we are serializing into sb3 files. It should be present consistently.

Actual Behavior

Describe what actually happens

Sometimes, specifically when adding backdrops from the library, the md5ext property is not included in the serialized project

1. Login to scratch and create a new project
2. Add a backdrop from the library.
3. Click "Save Now"
4. Click File -> Save to your computer, this is sb3 (1)
5. Reload the page having saved the project to the server.
6. Click File -> Save to your computer, this is sb3 (2).
7. Unzip both sb3 (1) and (2). Compare the way the backdrop is serialized. In (1) there is no md5ext property, but in (2) there is.

Instead of doing file -> save, you can also pull JSON directly from the project server in steps 3 and 5, just make sure you have forced a save now before doing it. You get the same result.

Operating System and Browser

e.g. Mac OS 10.11.6 Safari 10.0

Mac/Chrome

/cc @ericrosenbaum noticed this while working with a script to fetch assets from projects to generate the various libraries. @BryceLTaylor this may be relevant to what you are working on with the asset checker, although I'm not sure.