How to serve file for URL with a given prefix?

Context

I have been serving files via Vapor (v4) successfully with:

func configure(_ app: Application) throws {
   // ...
   app.middleware.use(FileMiddleware(publicDirectory: app.directory.publicDirectory))
   // ...
}

Problem

This attempts to map any/every URL that would match a file within publicDirectory.
How would one limit the "reach" of the FileMiddleware to only requests whose URL have a specific prefix? (say /files/)

As in, if I have /Public/foobar.txt, one would request it via GET /files/foobar.txt while GET /foobar.txt would NOT match anything.

Considered approaches

My understanding is that Middleware is applied server-wide (affects every "route") and not a subset ... so this wouldn't work.
Moving /Public/foobar.txt to /Public/files/foobar.txt is NOT a solution as I'm trying to limit URLs that can potentially get mapped to the file system...
Redirecting GET /files/foobar.txt to GET /foobar.txt is also not acceptable. Again, I need to limit the potential URLs which are mapped to the file system.

Solution

It seems that FileMiddleware will only work globally and not if attached to groups of routes as is usually the case with instances of middleware.

If you have a folder called Private in the project folder (i.e. the same folder holding your Public folder), then accessing files contained in it is straightforward:

public func configure(_ app: Application) throws {
    app.get("files", "**") { req -> Response in
        let filepath = req.parameters.getCatchall()
        let workPath = DirectoryConfiguration.detect().workingDirectory
        var fileURL = URL(fileURLWithPath: workPath)
            .appendingPathComponent("Private", isDirectory: true)
            .appendingPathComponent(filepath.joined(separator: "/"), isDirectory: false)
        return req.fileio.streamFile(at: fileURL.path)
    }
}

Assuming you are running this minimal project on localhost:8080, it will serve files via the URL:

http://localhost:8080/files/path/to/myFile.txt

EDIT

OP indicates flat files only. In the light of comment, made it work with arbitrary paths under Private/. I'll leave you to add action if file/path does not exist.