I just found that when you run optimization with PNG to JPG conversion active (or when you force the conversion manually). The new file gets a number (usually '1') in the filename.
So if you optimize the file Image.png, you will end with a new file with the name Image1.jpg
That's not good :( it brokes your "pretty" filenames that you put carefully adding that number.
And it makes not sense, because if you do the optimize over a file on the same format, for example if you optimize a JPG file (not changing format), the filename remains exactly the same, no changes...
I think (IMHO) that should be the default behaviour in all cases, keeping the filename without changes.