FIX: followup to a8d58c3b

It helps to include the files you intend to add

app/assets/javascripts/discourse/mixins/filter-mode.js.es6

@@ -0,0 +1,50 @@
+import Mixin from "@ember/object/mixin";
+import { computed } from "@ember/object";
+import Category from "discourse/models/category";
+
+export default Mixin.create({
+  filterModeInternal: computed(
+    "rawFilterMode",
+    "filterType",
+    "category",
+    "noSubcategories",
+    function() {
+      const rawFilterMode = this.get("rawFilterMode");
+      if (rawFilterMode) {
+        return rawFilterMode;
+      } else {
+        const category = this.get("category");
+        const filterType = this.get("filterType");
+
+        if (category) {
+          const noSubcategories = this.get("noSubcategories");
+
+          return `c/${Category.slugFor(category)}${
+            noSubcategories ? "/none" : ""
+          }/l/${filterType}`;
+        } else {
+          return filterType;
+        }
+      }
+    }
+  ),
+
+  filterMode: computed("filterModeInternal", {
+    get() {
+      return this.get("filterModeInternal");
+    },
+
+    set(key, value) {
+      this.set("rawFilterMode", value);
+      const parts = value.split("/");
+
+      if (parts.length >= 2 && parts[parts.length - 2] === "top") {
+        this.set("filterType", "top");
+      } else {
+        this.set("filterType", parts.pop());
+      }
+
+      return value;
+    }
+  })
+});