Add custom create handler for files

[Polleps]

When a class has a property with a classtype of file it might be confusing when a user is filling out the classes form.
If the user has not yet uploaded the file they want for the property they might use the create: <name> option in the dropdown, but this leads to the file form that can't be used to upload a file.

We should add a create handler for files that leads to an upload form instead of the default file form.

Alternatively we could also look into some kind of file picker UI that lists all files on the drive in a grid view and also allows the user to upload new files