Ambiguous use of 'appendingPathComponent' error

I am baffled as to what I should be using in Swift 3 for the following code:

func fileExist(_ filePath:AnyObject) -> Bool {
    let PdfPathWithFileName = filePath.appendingPathComponent(fileName as String)
    let fileManager = FileManager.default
    return fileManager.fileExists(atPath: PdfPathWithFileName)
}

I have tried changing the AnyObject to String but then I get another error:

'appendingPathComponent' is unavailable: Use appendingPathComponent on URL instead

Any ideas?

Edit:

String now exposes a method appendingPathComponent(_:). You no longer have to cast your String to an NSString. The answer below is no longer correct (And may have been out-of-date when I posted it.)

The method appendingPathComponent is a method of NSString. You need to make your parameter an NSString if you want to pass in a string. Otherwise pass in an NSURL