the jpeg image format plugin can not read jpg file without exif info or without exif orientation info. refer to the attachment.

The problem is:
1, in bool QJpegHandlerPrivate::read(QImage *image)
Only if "State==ReadHeader', then it will read the jpeg contents.

But
2, in bool QJpegHandlerPrivate::readJpegHeader(QIODevice *device)

if (exifOrientation == -1)
return false;

At that time, even if the file is a valid jpg, if it does not contain any exif infos or does not contain exifOrientation infos, it will retrun directly and leave "state" not set to "ReadHeader".

Here is a patch to fix it:
diff -Nur qtbase-opensource-src-5.5.0/src/gui/image/qjpeghandler.cpp qtbase-opensource-src-5.5.0n/src/gui/image/qjpeghandler.cpp
— qtbase-opensource-src-5.5.0/src/gui/image/qjpeghandler.cpp 2015-06-29 20:04:51.000000000 +0000
+++ qtbase-opensource-src-5.5.0n/src/gui/image/qjpeghandler.cpp 2015-08-13 20:41:32.187145848 +0000
@@ -900,7 +900,8 @@
size = QSize(width, height);

format = QImage::Format_Invalid;

• read_jpeg_format(format, &info);
  + if(read_jpeg_format(format, &info))
  + state = ReadHeader;

QByteArray exifData;

@@ -935,7 +936,6 @@
transformation = exif2Qt(exifOrientation);
}

• state = ReadHeader;
  return true;
  }
  else

if "file is a valid jpg file" and be supported, then set "state" to "ReadHeader".

And the jpeghandler codes still need more improvements, for example:
"readJpegHeader" had a "bool' retrun value but the value is never used.

Actually, the existence and use of "ReadHeader" state is awkward and almost useless.