I have seen many first-time users of MacPass open their *.kdbx file, enter the password and then select the *.kdbx file again as the keyfile. If the *.kdbx file is not actually a keyfile, This results in MacPass erroring with The database header hash is wrong which is technically correct, but confusing.

I would suggest displaying a warning when users select a *.kdbx file in the keyfile drop down, similar to login dialogs warning you when typing a password with caps lock enabled. The warning could explain what a keyfile is, e.g. The selected keyfile seems to be a KeePass database. If, in addition to the password, you're not using a dedicated keyfile to protect your KeePass database, you don't have to select anything here.