Hi @Webwerkplaats
The main problem is that WPFTS basically works with database entries in the wp_posts table, not with physical files. The files are the source of the search text only at the time of the indexing procedure, and thereafter the index only references the rows in the database table. It works this way because WPFTS is essentially an extension of the standard WordPress function, WP_Query, which searches wp_posts records.
What I might suggest in your case?
Since Simple File List Pro does not save information to WP Media Library, we need another plugin that will populate wp_posts with information about each file. In this case, with a small addon, the WPFTS plugin can index each file and display it in the search results (every file).
Another option is to index files that are embedded in the page using the [eeFSL] shotcode. But in this case it will be impossible to highlight a single file in the search results - instead a link to the page itself, where the list of files is embedded, will be displayed.
I need to understand exactly what you need in order to give further instructions or advice. Please let me know.