Unfortunately, no. Firefox for desktop does the same.

Thanks! I was hoping that there is an option in about:config.

The problem at hand is that .pgp somehow are not opened by OpenKeychain upon download, whereas .gpg files are.

Otherwise, Firefox on Android is attractive, especially the built-in PDF viewer. I hope this issue will be fixed.

What is the rationale? Make sure that file extensions match mime type? (security)

In the meantime, I am trying to figure out if it is possible to have OpenKeychain show up as handler for .pgp files.

Firefox for Android changes file extensions based on MIME type, like ".pgp" for "application/pgp-encrypted". There's no option to disable this, so you’ll need to manually rename the file after download or use a different download manager.

Thanks! I would just like to understand the rationale.

I don't even mind so much the change of the extension. What I do mind is that OpenKeychain is not listed as an option to decrypt .pgp files. With ".gpg" files, that issue does not exist.