DNG: exiv2 vs exiftool previewimage inconsistency about exiv2

[whatdoineed2do]

There appears to be another issue; when examinging the DNG file for previews, the LoaderTiff is used but it makes an assumption that all previews will have a "image/tiff" mime-type (as set in LoaderExifDataJpeg::getProperties()).

In the use case above, we are able to use exiftool to udpate the preview image using a jpeg - I do not know if this is to spec but at least Phil Harvey's commet notes such a DNG with JPEG preview image can be read correctly by MacOS.

[kmilos]

I wouldn't get hung up on the size per se, as they don't really disagree:

Exif.Image.StripByteCounts                   Long        1  72470

  | 12) PreviewImageLength = 72470

However, the preview not being detected as a JPEG might need looking into.

[kmilos]

## NOTICE image size reported as 188984 vs actual 188855 (+118bytes diff)
## BUT looking at -pt it CORRECTLY reports size

So the -pt reports the actual size of the JPEG byte stream, and that is in agreement and what matters. The +118 bytes is the size of the TIFF preview image one would extract - the extra bytes are for the TIFF container that wraps that JPEG payload.

Not sure if there's an actual bug here, just difference in behaviour: exiv2 decides to treat DNG/TIFF subimages as still valid TIFF images when possible, and doesn't necessary "unwrap" them unless they're specifically designated as preview/thumbnail images (using e.g. the JPEGInterchangeFormatLength tag instead of StripByteCounts, which in contrast seems to make LoaderTiff the priority loader).