Safari recognizes the meta information in the download and knows it is a GPX so offers apps that can handle GPX. Chrome, as best I can tell, only looks at the file extension and these lack that. Always use Safari for downloading GPX from web to Cachly.
I got it! I was using Chrome on my iPhone and that was the problem. I switched to Safari and it downloaded straight to Cachly and it works perfectly. Too bad I just finished the last Lab cache and am now sitting at dinner.
Cachly wins again!