Set File Filter Mode to Files and Folders.
Activate FlatView Mixed.
In the File Filter field enter a string expression.
Result: Only the files are filtered, although Filter Mode is set to Files and Folders!
This evidently is a side-effect of the FlatView mode, however it would be desirable that the File Filter also works in this situation (i.e. also on folders). Or is there another way to achieve this?
This is by (somewhat controversial) design. In Opus 8 a lot of people would switch to Flat View mode, filtered out some folders, and then complain or be confused because everything below the folders was filtered out as well. In Opus 9 Flat View was changed to ignore folder filters.
(There were also some technical problems which led to the decision, if I remember correctly.)
It's not ideal, but that's how it is right now. I usually end up doing a Find with an appropriate filter in the situations where I need to filter out folders, but that obviously results in a static list rather than a life one so it's not always suitable.