file-saver: Allow saving files directly (#16045)

As documented on https://github.com/eligrey/FileSaver.js/ saving a file
without using the Blob object should be possible

types/file-saver/file-saver-tests.ts

@@ -19,3 +19,12 @@ function testSaveAs() {
 
     saveAs(data, filename, disableAutoBOM);
 }
+
+/**
+ * @summary Test for "saveAs" function.
+ */
+function testSaveAsFile() {
+    const data = new File(["Hello, world!"], "hello world.txt" ,{type: "text/plain;charset=utf-8"});
+    
+    saveAs(data);
+}

types/file-saver/index.d.ts

@@ -1,6 +1,6 @@
 // Type definitions for FileSaver.js
 // Project: https://github.com/eligrey/FileSaver.js/
-// Definitions by: Cyril Schumacher <https://github.com/cyrilschumacher>
+// Definitions by: Cyril Schumacher <https://github.com/cyrilschumacher>, Daniel Roth <https://github.com/DaIgeb>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 
 /**
@@ -28,6 +28,14 @@ interface FileSaver {
         */
         disableAutoBOM?: boolean
     ): void
+
+    (
+        /**
+         * @summary File.
+         * @type {File}
+         */
+        data: File
+    ): void
 }
 
 declare var saveAs: FileSaver;