fix 404s for missing static files

test/pleroma/web/plugs/instance_static_test.exs

@@ -137,4 +137,47 @@ defmodule Pleroma.Web.Plugs.InstanceStaticTest do
     # It should be preserved because "image" is in the allowed_mime_types list
     assert content_type == "image/jpeg"
   end
+
+  describe "404s for missing files in static-only paths" do
+    test "returns 404 for non-existent static-only JSON files" do
+      conn = get(build_conn(), "/static/non-existent.json")
+
+      assert conn.status == 404
+      assert ["application/json"] = get_resp_header(conn, "content-type")
+      assert Jason.decode!(conn.resp_body) == %{"error" => "not found"}
+    end
+
+    test "returns 404 for non-existent static-only non-JSON files" do
+      conn = get(build_conn(), "/static/non-existent.txt")
+
+      assert conn.status == 404
+      assert conn.resp_body == "Not found"
+      assert ["text/plain"] = get_resp_header(conn, "content-type")
+    end
+
+    test "returns 404 for non-existent .css files" do
+      conn = get(build_conn(), "/static/non-existent.css")
+
+      assert conn.status == 404
+      assert conn.resp_body == "Not found"
+      # Verifies that we forced text/plain for the error body, even though the path was .css
+      assert ["text/plain"] = get_resp_header(conn, "content-type")
+    end
+
+    test "returns 404 for non-existent files without an extension" do
+      conn = get(build_conn(), "/static/non-existent")
+
+      assert conn.status == 404
+      assert conn.resp_body == "Not found"
+      assert ["text/plain"] = get_resp_header(conn, "content-type")
+    end
+
+    test "returns 200 (falls through to SPA) for non-static-only paths" do
+      # /some-route is NOT in static_only_files, so it should still fall through to the SPA.
+      conn = get(build_conn(), "/some-route")
+
+      assert conn.status == 200
+      assert ["text/html; charset=utf-8"] = get_resp_header(conn, "content-type")
+    end
+  end
 end