After I import a folder, there's often files that are left overs like extra covers, some text files and sometime pdfs. Is it possible to have beets import them as well?
Not quite yet! That's the idea behind "attachments" as described in this very popular feature request: https://github.com/beetbox/beets/issues/111
There's also a third-party plugin that accomplishes this most of the way: https://github.com/sbarakat/beets-copyartifacts