title_body: How to add a calibre bookmark txt to manifest
upvoted_answer: Just hit this myself trying to put a book on Smashwaords. It seems that browsing a book with Calibre adds an extra file called 'calibre_bookmarks.txt'. (Smashwords spots it, but the IDPF's Epub Validator doesn't!)
  
  The solution's simple enough:
  
  
  Change the book's extension from .epub to .zip. (Eg: mybook.epub ->
  mybook.zip) 
  Open the .zip file. 
  Delete the 'calibre_bookmarks.txt' file. 
  Save and close the .zip.
  Rename it back to .epub.
  
  
  Alternative method: Don't open your books in Calibre ;-)