feat: Load datatype and modality when building file context

[effigies]

This grew out of 60b5168 on #268.

From #268 (comment):

This is necessary to allow associations to use datatype or modality in their selectors. I would like to refactor datatype/modality handling, as that really doesn't need to be part of filenameIdentify stages. That will be a separate PR, when I have the time.

This is that PR.

I don't think this needs a changelog entry, as there is no user-facing consequence, given the current schema.