I am offering an ebook to my visitors in .epub format. I uploaded the .epub file to my server. However, when visitors download the file, it downloads as a .zip file rather than an .epub file. I know that an epub file is just a zipped file but my visitors shouldn't have to change the extension from zip to epub. It's on my server as an epub file so why does the extension change to zip upon download? How do I correct this?