Fix validation steps for image files

internal/topdata/topdata_test.go

@@ -8073,7 +8073,7 @@ func TestBuildAndPackageIncludesCompiled2DAAndTopAssets(t *testing.T) {
   "columns": ["Label"],
   "rows": [{"id": 0, "Label": "TEST_LABEL"}]
 }`+"\n")
-	writeFile(t, filepath.Join(root, "topdata", "assets", "gui", "testicon.tga"), "icon-data")
+	writeFile(t, filepath.Join(root, "topdata", "assets", "gui", "testicon.png"), "icon-data")
 
 	proj := testProject(root)
 	proj.Config.TopData.ReferenceBuilder = ""
@@ -8106,7 +8106,7 @@ func TestBuildAndPackageIncludesCompiled2DAAndTopAssets(t *testing.T) {
 		if key == "repadjust.2da" {
 			found2DA = true
 		}
-		if key == "testicon.tga" {
+		if key == "testicon.png" {
 			foundAsset = true
 		}
 	}
@@ -8114,13 +8114,31 @@ func TestBuildAndPackageIncludesCompiled2DAAndTopAssets(t *testing.T) {
 		t.Fatalf("expected repadjust.2da in top package")
 	}
 	if !foundAsset {
-		t.Fatalf("expected testicon.tga in top package")
+		t.Fatalf("expected testicon.png in top package")
 	}
 	if _, err := os.Stat(result.OutputTLKPath); err != nil {
 		t.Fatalf("expected packaged tlk output: %v", err)
 	}
 }
 
+func TestValidateProjectAcceptsPNGTopPackageAsset(t *testing.T) {
+	root := testProjectRoot(t)
+	mkdirAll(t, filepath.Join(root, "topdata", "data", "repadjust"))
+	mkdirAll(t, filepath.Join(root, "topdata", "assets", "gui"))
+	writeFile(t, filepath.Join(root, "topdata", "base_dialog.json"), "{}\n")
+	writeFile(t, filepath.Join(root, "topdata", "data", "repadjust", "base.json"), `{
+  "output": "repadjust.2da",
+  "columns": ["Label"],
+  "rows": [{"id": 0, "Label": "TEST_LABEL"}]
+}`+"\n")
+	writeFile(t, filepath.Join(root, "topdata", "assets", "gui", "testicon.png"), "icon-data")
+
+	report := ValidateProject(testProject(root))
+	if report.HasErrors() {
+		t.Fatalf("expected png topdata asset to validate, got %#v", report.Diagnostics)
+	}
+}
+
 func TestBuildPackageRequiresExistingCompiledOutput(t *testing.T) {
 	root := testProjectRoot(t)
 	mkdirAll(t, filepath.Join(root, "topdata", "data", "repadjust"))