There is no function available for attaching a file to a post, an essentially required basic function.
A workaround is to use the "Insert/Edit media" button for this, but this is (1) absolutely counter-intuitive and (2) tricky because it inserts a huge folder image into the post.
Please have a look at this post to get more insight into what I mean (and also into the fact that many people struggle with this issue).
Walter