Support other file types for fulltext search

[Accacio]

As discussed in #6188, not all files linked with an entry are used in fulltext search. A search for terms in a simple text file (.txt or .org) yields zero results.

Tested with
JabRef 5.5--2022-01-17--27a05c7
Linux 5.4.0-73-generic amd64
Java 16.0.2
JavaFX 17.0.1+1

and with
JabRef 5.6--2022-01-30--8ca6b7f
Windows 10 10.0 amd64
Java 16.0.2
JavaFX 17.0.2-ea+3
(@ThiloteE )