The base URL for WebDav into your mailbox is:
https://SERVER/service/dav/USER
This will prompt you for your username/password to authenticate. It will give you back a list of folders including briefcase and then some *special* folders. Like 'attachments' which will let you browse your entire mailbox's attachments by date, sender or type (image, audio, etc) See the screenshot below for examples. Please be aware this it not very optimized so some larger folders may timeout or take a long time to view. Also if you drag and drop a large number of files from your desktop it may take sometime for that to transfer as we index each one as they are added.