@phuongnh 

Please use the CSS code below: 

@media (max-width: 544px){
    #wpforo #wpforo-wrap .wpforo-post .wpforo-attached-file {
        max-width: 90%;
        min-width: 100%;
        display: inline-block;
        overflow: hidden;
        white-space: nowrap;
        overflow: hidden;
        text-overflow: ellipsis;
    }
}

Insert the CSS Code in the "Custom CSS Code" textarea in the wpForo > Settings > Colors & Styles admin page. 

Please don't forget to delete all caches and press Ctrl+F5 (twice) on the frontend before checking to reload the updated CSS file.

Also, try on incognito mode if you're checking on mobile devices. Because mobile browsers have a hard cache, you'll not be able to see the change immediately.