No user visible changes expected.
Add a function to query the full path for a file, so that asset files
can get the path via the asset representation and its new asset
identifier. This is designed to be a reliable way to locate an asset,
and using it is yet another step to rely less on the problematic file
browser code.
Also, previous code would build the full path manually in a few places,
which is good to deduplicate anyway.
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