I'm looking at buying a 1.4X Mark I teleconverter (I already have a Tamron one that doesn't register) and was just wondering if anyone knows if the Canon Mark I makes itself known in the metadata
I have a Canon 7D mk II, 70-200mm F2.8 II, and the 2x Extender III and when I import the images into Lightroom 5, I noticed that Lightroom Library mode does allow me to filter for using the lens with the extender. I get "EF70-200mm f/2.8L IS II USM +2x III" for the metadata and filtering options but when I go to Development mode, the Lens Corrections will only correct for the 70-200mm without the option of the 2x Extender. Now I know your asking about the 1.4x Extender with the 100-400mm but I suspect with the Canon lens and Canon Extender, the metadata in Lightroom will show the similar information as the 70-200 with 2x Extender.
The problem may not be with your existing teleconverter. It is most likely the way that Adobe products ignore most of the "maker notes" section of the EXIF information. The additional information about the TC and the resulting difference in focal length and aperture is almost certainly contained in the "maker notes" section rather than in the other standardized fields of the EXIF info.
Have you tried viewing the EXIF info via Canon's Digital Photo Professional to see if it properly shows up there?