When a JPEG has the Exif data, it does not have the JFIF information. When the Exif data are present, like for the orientation (-QTBUG-37946-), the reader should also read the image resolution from the Exif.

Attached testcases with 300x300 dpi resolution:

• jfif.jpg was saved with .net and has the JFIF data (Qt read and set the right resolution)
• exif.jpg was saved with Photoshop and has the Exif data (Qt leaves the default resolution, in my case 96 dpi).