The new forum software has changed things somewhat with respect to posting attachments.
At present to when you reply to a thread the (default) view is the standard editor which provides basic editing functionality only, to insert attachments you need to press the "Go Advanced" button.
Attachments can then be added by pressing the

button or the "Manage Attachments" button in additional options. This will take you into the new attachment management page from which you can add attachments using the "Add Files" button.
You can alter this behaviour to reduce the number of steps by updating your "setting" and selecting "Forum Setting".
From there you can change the Message Editor Interface to Enhanced Interface and disable the Enhanced Attachment uploading.
I'll look into it in more detail later and see if there are other ways to streamline the process.