Export MPMediaItem to file in Documents

ExportMPMediaItem.swift

/// Export MPMediaItem to temporary file.
///
/// - Parameters:
///   - assetURL: The `assetURL` of the `MPMediaItem`.
///   - completionHandler: Closure to be called when the export is done. The parameters are a boolean `success`, the `URL` of the temporary file, and an optional `Error` if there was any problem. The parameters of the closure are:
///
///   - fileURL: The `URL` of the temporary file created for the exported results.
///   - error: The `Error`, if any, of the asynchronous export process.

func export(_ assetURL: URL, completionHandler: @escaping (_ fileURL: URL?, _ error: Error?) -> ()) {
    let asset = AVURLAsset(url: assetURL)
    guard let exporter = AVAssetExportSession(asset: asset, presetName: AVAssetExportPresetAppleM4A) else {
        completionHandler(nil, ExportError.unableToCreateExporter)
        return
    }

    let fileURL = URL(fileURLWithPath: NSTemporaryDirectory())
        .appendingPathComponent(NSUUID().uuidString)
        .appendingPathExtension("m4a")

    exporter.outputURL = fileURL
    exporter.outputFileType = "com.apple.m4a-audio"

    exporter.exportAsynchronously {
        if exporter.status == .completed {
            completionHandler(fileURL, nil)
        } else {
            completionHandler(nil, exporter.error)
        }
    }
}

func exampleUsage(with mediaItem: MPMediaItem) {
    if let assetURL = mediaItem.assetURL {
        export(assetURL) { fileURL, error in
            guard let fileURL = fileURL, error == nil else {
                print("export failed: \(error)")
                return
            }

            // use fileURL of temporary file here
            print("\(fileURL)")
        }
    }
}

enum ExportError: Error {
    case unableToCreateExporter
}

Question posted in comments to http://stackoverflow.com/questions/37397581/how-add-int-vaue-to-the-alamofire-upload-parameters/37397828#comment69244352_37397828

Adapted from http://stackoverflow.com/a/40929355/1271826