The Digests and Subscriptions link in the menu at the top of the page will take you to a page that describes the Digests feature and the Subscriptions feature. Links on that page will take you to separate pages where you may configure your settings related to these distinct features.
All registered users may post messages by email irrespective of their subscription settings. There is a special email address for each forum, a list of which may be found on the Subscriptions page. Note, particularly, that you must use the same email address when sending posts as you used to register at this forum.
Messages posted by email may also include attachments. However, there is a finite set of extension types that are allowed as attachments and there is a limit on the size of attachments as well. These limits may vary among the various fora but you can get a general idea of the limits here.
You'll find a link in the 'Add an Attachment' section of the 'New Topic' page of a forum that gives the attachment limitations for that particular forum.