How you store your ebook bonus extras?

[yonkyunior]

Nowadays, i have been reorganizing my collection.
Giving a tags and others, but many of my ebook in TextBooks, Magazine comes with extras, such as examples codes, posters, etc.
When i bought a book that comes with extras like these or the book with bonus CD or something, i wanna keep it more organized in one place.

so i have ebook file (epub,mobi, PDF)
and extras file (zip)

Now, i just directly add the zip file to the book and merge it.
so the version of ebook now have (ePub, mobi, PDF, and ZIP)
i dont like it, but it just works for now.
I dont think any other replacement method for this case.
I want have extras field that linked to the zip so the ZIP no longer exist as a book version

Do you have other method for organizing like this case?
Thanks.