Look in your ACP, it should tell you if any errors have been logged.
For the browser, open the console first,
from the menu: select "Browser Console" from the Web Developer submenu in the Firefox Menu (or Tools menu if you display the menu bar or are on OS X)
from the keyboard: press...
This may still be work but maybe a bit buggy, https://xenforo.com/community/resources/ultimate-attachment-manager.129/ or this might do it, https://xenforo.com/community/resources/attachment-improvements-by-xon.5042/