save previews for extra networks in the selected format

modules/ui_extra_networks.py

@@ -144,6 +144,9 @@ class ExtraNetworksPage:
         """
 
         preview_extensions = ["png", "jpg", "webp"]
+        if shared.opts.samples_format not in preview_extensions:
+            preview_extensions.append(shared.opts.samples_format)
+
         potential_files = sum([[path + "." + ext, path + ".preview." + ext] for ext in preview_extensions], [])
 
         for file in potential_files: