How to add Artrage to “open in” options in Dropbox for iPad

[Lottie]

Try as I might, on my new iPad Pro I can’t see how to add Artrage to the options available in Dropbox for sending to my device or opening a file.
I need to send hundreds of ptg files. I tried in the gallery to import files from Dropbox, but after only 2 or 3 images, everything ground to a halt!
I’m sure there must be a simple way to add Artrage to the list of options. It appears in Dropbox on my other iPad, but I can’t remember how that was set up. Can anyone help?