There is two ways to access the window "Manage external file types":

1. Options -> Manage external file types
2. Options -> Preferences, tab External programs, button "Manage external file types"

Considering the management of external file types is seldom, I suggest to keep only the second way. Moreover, it helps in having all the preferences in the Preferences window.

JabRef 5.2--2020-10-07--4b1b7b4
Linux 4.9.0-13-amd64 amd64
Java 15