title: S3 limit files only to certain types
body: <p>I am using direct browser uploads using presigned urls to allow file uploads. so far I am using </p>
  
  <pre><code>    $s3 = App::make('aws')-&gt;createClient('s3');
  $command = $s3-&gt;getCommand('PutObject', [
              'Bucket' =&gt; 'reminderuploadstest',
              'Key' =&gt; "test.txt",
          ]);
  
         echo ( (string)$s3-&gt;createPresignedRequest($command,'+100 minutes')-&gt;getUri());
  </code></pre>
  
  <p>I would like to know how to limit the file types that can be uploaded using this url. I want the user to be able to upload only images(e.g. jpg,png). 
  Is it possible for amazon to check if it is a 'valid' image ?(i.e. not just extension). If its not possible , what can I do to ensure the user uploads only images. It is a requirement that I upload to s3 directly and not use some online validation and then proxy it over to s3</p>