     
     public func mergeClips(usingPreset preset: String, completionHandler: @escaping NextLevelSessionMergeClipsCompletionHandler) {
         self.executeClosureSyncOnSessionQueueIfNecessary {
-            let fileType = self.fileType()
-            let fileExtension = self.fileExtension(fileType: fileType)
+            let fileType = self.fileType
+            let fileExtension = self.fileExtension
             let filename = "\(self.identifier)-NL-merged.\(fileExtension)"
 
             let outputURL: URL? = NextLevelClip.clipURL(withFilename: filename, directory: self.sessionDirectory)


extension NextLevelSession {
    
    internal func fileType() -> String {
        return AVFileTypeAppleM4A
    }
    
    internal func fileExtension(fileType: String) -> String {
        return "m4a"
    }
    
    internal func nextFileURL() -> URL? {
        let fileType = self.fileType()
        let fileExtension = self.fileExtension(fileType: fileType)
        let filename = "\(self.identifier)-NL-clip.\(self.sessionClipFilenameCount).\(fileExtension)"
        if let url = NextLevelClip.clipURL(withFilename: filename, directory: self.sessionDirectory) {
            self.removeFile(fileUrl: url)
            self.sessionClipFilenameCount += 1

    
    override init() {
        self.sessionIdentifier = NSUUID().uuidString
        self.fileType = AVFileTypeMPEG4
        self.fileExtension = "mp4"
        
        self.sessionDirectory = NSTemporaryDirectory()
        self.sessionClips = []
        self.sessionClipFilenameCount = 0
