Android 调用openAssetFileDescriptor时未找到文件异常

我正在像这样读取位图。我从画廊里挑了一个，得到了一个例外。请帮我解决这个问题

// Read bitmap
    public Bitmap readBitmap(Uri selectedImage) {
        Bitmap bm = null;
        BitmapFactory.Options options = new BitmapFactory.Options();
        options.inSampleSize = 5;
        AssetFileDescriptor fileDescriptor =null;
        try {
            fileDescriptor = this.getContentResolver().openAssetFileDescriptor(selectedImage,"r");
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
        finally{
            try {
                bm = BitmapFactory.decodeFileDescriptor(fileDescriptor.getFileDescriptor(), null, options);
                fileDescriptor.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return bm;
    }   

日志跟踪：

06-13 14:05:11.470: W/System.err(20799): java.io.FileNotFoundException: No such file or directory
    06-13 14:05:11.470: W/System.err(20799):    at android.os.Parcel.openFileDescriptor(Native Method)
    06-13 14:05:11.470: W/System.err(20799):    at android.os.ParcelFileDescriptor.open(ParcelFileDescriptor.java:117)
    06-13 14:05:11.480: W/System.err(20799):    at android.content.ContentResolver.openAssetFileDescriptor(ContentResolver.java:609)
    06-13 14:05:11.480: W/System.err(20799):    at com.example.Main.readBitmap(Main.java:416)
    06-13 14:05:11.480: W/System.err(20799):    at com.example.Main.addContents(Main.java:315)
    06-13 14:05:11.480: W/System.err(20799):    at com.example.Main.sendMMS(Main.java:231)
    06-13 14:05:11.480: W/System.err(20799):    at com.example.Main.access$16(Main.java:228)
    06-13 14:05:11.480: W/System.err(20799):    at com.example.Main$AsyncNetworkOperation.doInBackground(Main.java:397)
    06-13 14:05:11.490: E/Trace(23274): error opening trace file: No such file or directory (2)
    06-13 14:05:11.490: D/ActivityThread(23274): setTargetHeapUtilization:0.25
    06-13 14:05:11.490: D/ActivityThread(23274): setTargetHeapIdealFree:8388608
    06-13 14:05:11.490: D/ActivityThread(23274): setTargetHeapConcurrentStart:2097152

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目录是什么？file:///mnt/sdcard/DCIM/Camera/2013-06-08%2022.18.06.jpgWell 我从来没有做过你想做的事，但我想你需要一些权限才能从图库访问你的照片。