Is the Additional Options bar showing below the message box when you make a post? Or the paper clip /attachment icon above near the fonts, tools and options?

You may have to click the arrows on the side of the bar top expand that options area.

If the options are showing for you then use this part-
Attach Files
Valid file extensions: bmp doc gif jpe jpeg jpg pdf png psd txt zip

[How do I attach a file to a post? (from FAQ's)

To attach a file to your post, you need to be using the main 'New Post' or 'New Thread' page and not 'Quick Reply'. To use the main 'New Post' page, click the 'Post Reply' button in the relevant thread.

On this page, below the message box, you will find a button labelled 'Manage Attachments'. Clicking this button will open a new window for uploading attachments. You can upload an attachment either from your computer or from another URL by using the appropriate box on this page. Alternatively you can click the Attachment Icon to open this page.

To upload a file from your computer, click the 'Browse' button and locate the file. To upload a file from another URL, enter the full URL for the file in the second box on this page. Once you have completed one of the boxes, click 'Upload'.

Once the upload is completed the file name will appear below the input boxes in this window. You can then close the window to return to the new post screen.

What files types can I use? How large can attachments be?

In the attachment window you will find a list of the allowed file types and their maximum sizes. Files that are larger than these sizes will be rejected. There may also be an overall quota limit to the number of attachments you can post to the board.]