Maybe
this has something to do with it?
Quote:
The Barnes & Noble Fixed-layout Format
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DRP is essentially an ePub: it’s a zip file with the normal ePub structure, including an OPF with a manifest, spine, etc. In essence, a DRP contains two categories of items: images of pages rasterized from the incoming PDF, and the text (and images, if desired) of “articles” contained in the magazine (XHTML). There are a couple of ways in which a DRP deviates from the ePub standard: 1) the format & reading system allow images to be top-level items in the spine, and 2) the addition of a replica map (described in detail later), mapping page images to XHTML content (and vice versa). “ArticleView™” presents the text of an article plus a headline, byline and one or more images in an HTML view. In our implementation, the article view “floats” above the page image, with some of the underlying page image visible along the sides of the article view.
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But depending on
part of the filename to clue in the reader software as to what format it is dealing with seems kludgy at best - and 1970's kludgy at that.