Fix: custom document model not working with wagtail grapple

[fabienheureux]

Current version of Grapple does not seem to work with custom document model.
This fixes it

• Fix the issue
• Add tests

Edit: I added a custom document model in the example project in order to ensure that a custom document model works with Grapple.
Sorry for all the commits, this is the first time I worked with the example project and a few things went wrong because I did not install pre-commit hooks 😅

Also: I had to port this fix in order to fix example project tests: https://github.com/wagtail/wagtail/pull/6559/files
Let me know if I should rather fix this in another PR.

[zerolab]

@fabienheureux another omission in tests! Can you add a test to confirm your custom doc model is picked up?

[fabienheureux]

@zerolab I updated the code a bit to make the document type file look closer to the image type file.
It helped me debugging and it will make the codebase more consistent, what do you think ?

And I added a test 😅 (sorry for the delay.....).

[zerolab]

Thank you @fabienheureux, merged with a tweak