Hello. I am using Calibre as my ebook management program and I have set SumatraPDF as external epub viewer. It handles epub files quickly and fine.
Calibre can convert an epub file to another epub file changing its file extension to .original_epub; this opens the possibility of storing two versions of the same ebook.
However, files having the .original_epub extension cannot be read by SumatraPDF, unless going through the hassle of copying it to a temp folder and changing the “.original_epub” extension back to “.epub”. This means that the problem lies in recognizing the file extension rather than a file format problem.
For us compulsive epub readers it would be a must if you could add the feature of being able to read these .original_epub files, perhaps by parsing the file extension to check for the presence of the string “epub”.