Why is local history turned off for files detected via FileTypeDetector?

My plugin has a FileTypeDetector to detect the file type of files without extension, based on the content / shebang line.

A user reported that local history is disabled for such files and I can confirm this.
I would like to enable the local history for detected files.
Internally, IdeaGateway.isVersioned(VirtualFile, boolean) returns false because WorkspaceFileIndex.isInContent(VirtualFile) reports false for this file, even though it’s in the local file system and it‘s inside a content root.

What’s the reason that local history is turned off and how could I turn it on?

Thank you!