Unfortunately that feature is not currently available on Blog.com. We will take it into consideration for future releases.

You can only upload music files and embed them in you blog – http://support.blog.com/index.php?/Knowledgebase/Article/View/54/12/adding-music-to-your-posts

If you want your readers to download the files, I’m afraid you’ll have to host them in some external service (Dropbox, for instance) and then link to the files in your blog posts.