[Astraport]

Использовать FileStream.

 var fileLocal:File = File.applicationStorageDirectory.resolvePath(pathLocal);  
var fileStream:FileStream = new FileStream();  
fileStream.addEventListener(Event.CLOSE, fileClosed);
fileStream.openAsync(fileLocal, FileMode.WRITE);  
fileStream.writeBytes(fileData, 0, fileData.length);  
fileStream.close();