Fix medium plugin handling

src/plugin.rs

@@ -226,7 +226,9 @@ impl Plugin {
     }
 
     pub fn is_medium_plugin(&self) -> bool {
-        self.is_medium_flag_set()
+        // Light plugins saved by the Creation Kit have both the light and
+        // medium flags set.
+        self.is_medium_flag_set() && !self.is_light_flag_set()
     }
 
     pub fn is_override_plugin(&self) -> bool {
@@ -1794,6 +1796,16 @@ mod tests {
             assert!(plugin.is_medium_plugin());
         }
 
+        #[test]
+        fn is_medium_plugin_should_be_false_for_a_plugin_with_the_medium_and_light_flags_set() {
+            let mut plugin = Plugin::new(
+                GameId::Starfield,
+                Path::new("testing-plugins/Starfield/Data/Blank.small.esm"),
+            );
+            assert!(plugin.parse_file(true).is_ok());
+            assert!(!plugin.is_medium_plugin());
+        }
+
         #[test]
         fn is_override_plugin_should_be_true_for_a_plugin_with_the_override_flag_set_and_at_least_one_master_and_no_light_flag(
         ) {