FileManager: Replace with copy of file

FileManager_Extensions.swift

extension FileManager {
    func replaceWithCopyOfFile(at:URL, with:URL) {
        do {
            let url = try self.url(for: .itemReplacementDirectory, in: .userDomainMask, appropriateFor: with.deletingPathExtension(), create: true)
            try self.copyItem(at: with, to: url.appendingPathComponent(with.lastPathComponent))
            
            let alert = NSAlert()
            alert.messageText = "Replace \"\(at.lastPathComponent)\" in \"\(at.pathComponents[at.pathComponents.count - 2])\" with new file?"
            alert.addButton(withTitle: "OK")
            alert.addButton(withTitle: "Cancel")
            
            if alert.runModal() == NSAlertFirstButtonReturn {
                _ = try FileManager.default.replaceItemAt(at, withItemAt: url.appendingPathComponent(with.lastPathComponent))
            }

            // removes whole temporary directory as a clean up
            try self.removeItem(at: url)         
        }
        catch {
            // error
            print("unknown error")
        } 
    }
}