WebUI: simplify attribute usage

The `multiple` attribute is a boolean value and its presence equals to
`true`.

src/webui/www/private/upload.html

@@ -14,7 +14,7 @@
     <iframe id="upload_frame" name="upload_frame" class="invisible" src="about:blank"></iframe>
     <form action="api/v2/torrents/add" enctype="multipart/form-data" method="post" id="uploadForm" style="text-align: center;" target="upload_frame" autocorrect="off" autocapitalize="none">
         <div style="margin-top: 25px; display: inline-block; border: 1px solid lightgrey; border-radius: 4px;">
-            <input type="file" id="fileselect" name="fileselect[]" multiple="multiple" />
+            <input type="file" id="fileselect" name="fileselect[]" multiple />
         </div>
         <fieldset class="settings" style="border: 0; text-align: left; margin-top: 12px;">
             <table style="margin: auto;">