I assume you are talking about the bug where this drop-down won't open after you first upload a file.
It's a bug in vBulletin, and we hope they will fix it. It's definitely annoying.
The workaround is to click the
Manage Attachments button under "Additional Options" to open the Manage Attachments window an extra time. Once you close that window, the drop-down works again.
For those who don't know, before you attach any files, you can bring up the attachments window by clicking the paperclip & triangle or by clicking the
Manage Attachments button under "Additional Options."
Once you have uploaded a file, and gotten past the bug by reopening and closing the Manage Attachments window, you can click the paperclip or triangle to open the drop-down. In the drop-down you can click the heading to go to the separate window or click a filename to put the image inline in your post. We ask that you do not put large images in-line, as a courtesy to other users.
For more information, see
Inline Attachments in the Mac Guides.