Just delete the surplus ".jar" extension. Basically what's happened is that the browser has said:
Hmmm - I don't know what a ".epub" file is - let's look inside it and see if we recognise it. Hey - it looks like a ZIP archive. Now what programs are installed on this computer that handle ZIP files? I know! A Java Archive file is a ZIP file, so let's give the file a ".jar" extension!
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