Add extension to filename when using withFilename()

Update `handleFilename()` to append appropriate file extension when `withFilename()` is used. The extension will continue be appended when `askForFilename()` is used.

src/Concerns/WithFilename.php

@@ -63,7 +63,7 @@ protected function handleFilename(ActionRequest $request): void
     {
         $fields = $request->resolveFields();
 
-        if ($filename = $fields->get('filename')) {
+        if ($filename = $fields->get('filename', $this->filename)) {
             if (!Str::contains($filename, '.')) {
                 $filename .= '.' . $this->getDefaultExtension();
             }