The mimetype application/epub+zip already exists in the share-mime-info package, so adding the line
MimeType=application/epub+zip;
to the desktop file should be enough.
I also think that that a separate desktop file should be created for the e-book viewer, as it can be usefil to start it indepently from the main
calibre progam.
As a workaround, you can create a ~/.local/share/applications/userapp-ebook-viewer.desktop with the following content:
[Desktop Entry]
Type=Application
Name=calibre-ebook-viewer
Comment=Calibre Ebook Viewer
Exec=ebook-viewer
Icon=/usr/share/calibre/images/viewer.png
Categories=Office;Database;FileTools;Utility;Viewer;Qt;
MimeType=application/epub+zip;
Type=Application
Name=calibre-ebook-viewer
Comment=Calibre Ebook Viewer
Exec=ebook-viewer
Icon=/usr/share/calibre/images/viewer.png
Categories=Office;Database;FileTools;Utility;Viewer;Qt;
MimeType=application/epub+zip;
Then, there are two ways that I know of to set the default applications (not sure which one is the recommended one).
Both command assume that you are in ~/.local/share/application and not root.
update the local defaults.list with:
xdg-desktop-menu install userapp-ebook-viewer.desktop
or alternatively, use the following to update the default in your local mimeapps.list:
xdg-mime default userapp-ebook-viewer.desktop application/epub+zip
