Opened the tag dialog ("Edit track details") for a file in the local collection.
The button is enabled now, clicking it successfully opens the containing folder in dolphin.
The "Location" text field still shows "/path/to/folder/file" (without "file://").
Added a remote podcast to the playlist, and opened the tag dialog.
The button stays disabled, the text field still correctly shows the Url including the scheme ("https://...") in my case).