I would like to print with Canon Platine Fiber rag. That paper comes with an am1x file with custom media types. I used the Canon Media Configuration Tool (MCT) to add the am1x file to my PRO-300 printer. It appears that the am1x file was installed correctly—it is listed under "Custom" in MCT for my PRO-300.
However, when I go to print, I do not see a "Custom" option in Quality & Media > Media Type, which is where it should appear: