How to check a blob file is a image in Laravel?

I'm using Laravel 5.3, I crop a image with javascript and upload it with ajax. The cropped image is changed to a blob file when uploading.

But in backend,How to check the blob file is a image?

    public function changeAvatar(Request $request)
    {
        $user = \Auth::user();
        $blob = $request->croppedImage;
        $destinationPath = 'images/uploads/';
        $fileName = $user->id . '_' . time() . '.png';
        $file = file_put_contents($destinationPath.$fileName, $blob);
        
    
         $input = array('image' => $file);
         $rules = array(
             'image' => 'image'
         );
         $validator = \Validator::make($input, $rules);
         if ( $validator->fails() ) {
             return \Response::json([
                 'success' => false,
                 'errors' => $validator->getMessageBag()->toArray()
             ]);
    
         }
    
        $user ->avatar = '/'.$destinationPath.$fileName;
        $user ->save();
    
        return \Response::json([
            'success'=>true,
            'avatar'=>asset($destinationPath.$fileName),
        ]);
    }

the file is a image,but the error is always like this:

    {"success":false,"errors":{"image":["image must be an image."]}}