How to export file with AVAssetExportPresetPassthough flag in the exportSession?

Question

I want to use the AVAseetExportSession class to export the file from ipod liabrary to appliation temp.

AVAssetExportSession *exporter = [[AVAssetExportSession alloc]
                               initWithAsset: fileAsset
                               presetName: AVAssetExportPassthrough];

if i want to you the AVAssetExportPassthrough flag to initialize the exporter,which file type i should specify to the exporter.outputFileType ?

once i specify AVFileTypeMPEG4 to the exporter.outputFileType, but the export session always been to the AVAssetExportSessionStatusFailed status.