I posted this question more than a month ago and i have found an answer to it, Here is the eclipse.desktop file which needs to be added in the /usr/share/applications directory
[Desktop Entry]
Encoding=UTF-8
Name=Eclipse
Comment=Eclipse IDE
Exec=eclipse %f
Icon=/opt/eclipse/icon.xpm
Terminal=false
Type=Application
Categories=GNOME;Application;Development;
StartupNotify=true
Adding %f after 'eclipse' added eclipse in the list of 'other applications' and then i 'set it as default' for css files.