importing song as mp3 from the ipod music library to the App

I am making an audio app. In which I have pick a song from iPhone device audio library then I want to save it to my app document folder as in mp3 format. i tired alot of codes, but they import the file in m4a format, and when i rename it to mp3, the album art and all the meta data vanished from the file.i used the below code , but it import the file as in m4a format.

NSString *songTitle = [item valueForProperty:MPMediaItemPropertyTitle];
NSString *artist = [item valueForProperty:MPMediaItemPropertyArtist];
NSString *album = [item valueForProperty:MPMediaItemPropertyAlbumTitle];
MPMediaItemArtwork *artwork = [item valueForProperty:MPMediaItemPropertyArtwork];
UIImage *artworkImage = [artwork imageWithSize:CGSizeMake(250.00, 250.00)];
AVMutableMetadataItem *artistMetadata = [[AVMutableMetadataItem alloc] init];
artistMetadata.key = AVMetadataiTunesMetadataKeyArtist;
artistMetadata.keySpace = AVMetadataKeySpaceiTunes;
artistMetadata.locale = [NSLocale currentLocale];
artistMetadata.value = artist;

AVMutableMetadataItem *albumMetadata = [[AVMutableMetadataItem alloc] init];
    albumMetadata.key = AVMetadataiTunesMetadataKeyAlbum;
    albumMetadata.keySpace = AVMetadataKeySpaceiTunes;
    albumMetadata.locale = [NSLocale currentLocale];
    albumMetadata.value = album;

    AVMutableMetadataItem *songMetadata = [[AVMutableMetadataItem alloc] init];
    songMetadata.key = AVMetadataiTunesMetadataKeySongName;
    songMetadata.keySpace = AVMetadataKeySpaceiTunes;
    songMetadata.locale = [NSLocale currentLocale];
    songMetadata.value = songTitle;

    AVMutableMetadataItem *imageMetadata = [[AVMutableMetadataItem alloc] init];
    imageMetadata.key = AVMetadataiTunesMetadataKeyCoverArt;
    imageMetadata.keySpace = AVMetadataKeySpaceiTunes;
    imageMetadata.locale = [NSLocale currentLocale];
    imageMetadata.value = [NSData dataWithData:UIImagePNGRepresentation(artworkImage)]; //imageData is NSData of UIImage.

    NSArray *metadata = [NSArray arrayWithObjects:artistMetadata, albumMetadata, songMetadata, imageMetadata, nil];

    //convert MPMediaItem to AVURLAsset.
    NSURL *assetURL = [item valueForProperty:MPMediaItemPropertyAssetURL];
    AVURLAsset *songAsset = [AVURLAsset URLAssetWithURL:assetURL options:nil];

    //get the extension of the file.
    NSString *fileType = [[[[assetURL absoluteString] componentsSeparatedByString:@"?"] objectAtIndex:0] pathExtension];

    //init export, here you must set "presentName" argument to "AVAssetExportPresetPassthrough". If not, you will can't export mp3 correct.
    AVAssetExportSession *export = [[AVAssetExportSession alloc] initWithAsset:songAsset
                                                                 presetName:AVAssetExportPresetPassthrough];

    NSLog(@"export.supportedFileTypes : %@",export.supportedFileTypes);
    //export to mov format.
    export.outputFileType = @"com.apple.quicktime-movie";
    export.metadata = metadata;

    NSString *extension = (__bridge  NSString *)UTTypeCopyPreferredTagWithClass((__bridge  CFStringRef)export.outputFileType, kUTTagClassFilenameExtension);

    NSLog(@"extension %@",extension);
    NSString *path = [NSString stringWithFormat:@"%@/%@.%@",self.folder.fullPath, songTitle, extension];

    NSURL *outputURL = [NSURL fileURLWithPath:path];
    export.outputURL = outputURL;
    [export exportAsynchronouslyWithCompletionHandler:^{
        if (export.status == AVAssetExportSessionStatusCompleted) {
        }
        else
        {
            NSLog(@"%@",export.error);
        }
    }];