Replace true with multiple

"true" is a wrong attribute value in multiple

app/partials/desktop/im.html

@@ -200,12 +200,12 @@
                       <button type="submit" class="btn btn-md im_submit" my-i18n="im_submit_message"></button>
 
                       <div class="im_attach pull-left">
-                        <input type="file" class="im_attach_input" size="28" multiple="true" title="{{'im_attach_file_title' | i18n}}" />
+                        <input type="file" class="im_attach_input" size="28" multiple="multiple" title="{{'im_attach_file_title' | i18n}}" />
                         <i class="icon icon-paperclip"></i>
                       </div>
 
                       <div class="im_media_attach pull-left">
-                        <input type="file" class="im_media_attach_input" size="28" multiple="true" accept="image/*, video/*, audio/*" title="{{'im_media_attach_title' | i18n}}"/>
+                        <input type="file" class="im_media_attach_input" size="28" multiple="multiple" accept="image/*, video/*, audio/*" title="{{'im_media_attach_title' | i18n}}"/>
                         <i class="icon icon-camera"></i>
                       </div>