The open with menu is limited to fbreader and adobe.
If you want to use another app to open your books you have to set it as the first one in extensions.cfg.
Example:
Quote:
epub:@EPUB_file:1:cr3-pb.app,AdobeViewer.app,fbreader.app:ICON_EPUB
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Here's the recommended FW5 extensions.cfg for coolreader:
github.com