FileTypes: prevent picking folder as preferred app

* I'm not sure why this was changed back in 2006, the file panel is
reused in various cases but I don't see one where selecting a folder
would make sense.
* If I missed something, then different file panels should be used for
the different cases.

Fixes #8805.

src/preferences/filetypes/FileTypes.cpp

@@ -180,7 +180,7 @@ FileTypes::FileTypes()
 	fTypeWindowCount(0)
 {
 	fFilePanel = new BFilePanel(B_OPEN_PANEL, NULL, NULL,
-		B_FILE_NODE | B_DIRECTORY_NODE, false);
+		B_FILE_NODE, false);
 }