gment is not edited when writing information if no EXIF tags are being
changed (eg. if only XMP or IPTC information is being edited).  In this case,
the preview image pointers could be invalidated because the length of the data
between the EXIF segment (which comes near the start of the file) and the
preview image (at the end of the file) is likely to change.  ExifTool gets
around this when reading JPEG images by looking for the preview at the end of
the file and updating pointers if necessary, but the prev