Add leader fallback for worker file imports

[stxue1]

Resolves #5135

Changelog Entry

To be copied to the draft changelog by merger:

• When importing on workers, fallback to import files on the leader when file sizes are not obtainable. Related to (Add filesize sniffing and parallelize importing on jobs #5133).

Reviewer Checklist

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    contrib/admin/test-pr otheruser theirbranchname issues/XXXX-fix-the-thing
    

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Merger Checklist

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[adamnovak]

This looks pretty good, but I think one of the new functions is missing a docstring, and I'm not sure about the clarity of some of the variable/function names.

[adamnovak]

This isn't really mapping to file data (bits in the file), right? This is metadata consisting of file size and... something.

Maybe this should be renamed and have a comment explaining what's actually in the values? Or an inline type hint and not just auto-typing?

[adamnovak]

I think this is missing a docstring.

It also might need a better name. It sounds like it is going to e.g. convert files from DOC to PDF or something, and really it is going to fill in Toil file IDs and replace URLs with other URLs.