How can I add zip, epub and mobi file types for MarketPress uploads?

I have a multisite installation (etsy style) where my multiple sub-sites are licensed storefronts. The managers of these storefronts will need to upload ebook format files for purchase.

How can I add zip, epub and mobi extensions to the acceptable upload file types in Wordpress so that my MarketPress storefronts can present their files for sale?