Another customer mentioned the issue with file extensions. There are a few things going on here.
Nitro uses .xmp for all XMP files. If it’s a proprietary RAW (that is, not a DNG), then it writes with just the XMP extension. If it’s a JPEG, DNG, etc., then it uses the underscore.
It does that to ensure there cannot be a conflict between a proprietary RAW and a JPEG with the same base name. This can happen a couple of ways. One is RAW+JPEG. Another is a RAW + an exported JPEG stored in the same directory. Or, the JPEG is first imported and then the RAW from a R+J pair. Also, this makes it simpler for the app to know what file goes with which.
The terminology of “.jpg.xmp” is attempting to follow a convention of “two extensions”, but that is supposed to be used in a particular case – when the storage is “layered”. For example, TAR and GZIP. That is usually done as x.tar.gz because it’s a TAR file that has been gzipped. Once you unzip it, you have x.tar.
But that’s not true for a file named “.jpg.xmp.” The JPEG is not stored inside an XMP container – it’s not a layered model.
I will probably add a preference at some point to let sidecars be named with the double extension, though I don’t think it’s correct. If I do that, I would update the “Migrate Sidecar” feature to adjust the names of existing sidecars in a directory. I’m not inclined to have the app search for both files and try to figure out which one to use though. Having two files containing similar data always leads to problems.