The iOS environment is complicated. AxCrypt actually can’t open anything except AxCrypt-files. It then delegates to iOS to determine the appropriate application to open the decrypted file. This does require the app itself to register that it can open the file in question.
I’m not sure how Keepass does, perhaps it doesn’t register itself?
If you use a file manager to browse to the db file, does your phone offer to open it in the Keepass app? Exactly what Keepass iOS app are you using? We’ll try this out, but it would be nice to know what app you’re using as there seems to be several.
Speaking on the subject of Keepass, you might want to try our online password manager for the same purpose…