TOOLS/lua/autoload: fix adding more files at end of auto-playlist

Fixes regression in dfd8a5f that made autoload not add more files
at the end of the current playlist if playlist was made by the
script.

This still prevents loading the script if more than one (media or
playlist) file was manually added.

TOOLS/lua/autoload.lua

@@ -47,9 +47,12 @@ function find_and_add_entries()
     if #dir == 0 then
         return
     end
-    local isplaylist = mp.get_property("playlist-count")
-    if #isplaylist > 1 then
+    local pl_count = mp.get_property_number("playlist-count", 1)
+    if (pl_count > 1 and autoload == nil) or
+       (pl_count == 1 and EXTENSIONS[string.lower(get_extension(filename))] == nil) then
         return
+    else
+        autoload = true
     end
 
     local files = mputils.readdir(dir, "files")