I found this CSS in another post somewhere here in the forums. Will hide the ‘attachments’ folder in the sidebar. Just put this in a CSS file in your snippets folder and activate it in Settings > Appearance. That’s also where you can click on the folder icon to open the snippets folder in the first place.
div[data-path='attachments'],
div[data-path='attachments'] + div.nav-folder-children
{
display: none;
}