The `AssetRepresentation` should now be available directly in context
where previously the `AssetHandle` type was required.
By adding an `"asset"` context member to the file browser, this should
avoid any behavior change. (Previously we would get the asset from
context by constructing a asset handle from the "active_file" context
member. So make sure "asset" is available wherever "active_file" is.)
This is another important step towards removing `AssetHandle`, see
blender/blender#108806.
3f9c943243