Attachment count is not updated when the Attachments tab is loaded

Follow these steps to reproduce:

• open the same wiki page on two browser tabs (but don't open the Attachments tab)
• the header of the Attachments tab should show the same number on both (0)
• open the Attachments tab on one of the browser tabs and upload a file; notice that the attachments count is not updated, this is XWIKI-10096
• go to the other browser tab and open the Attachments tab; the tab header has not been updated (still shows 0 although you can see one attachment listed)