disable "show all extensions" in Finder preferences.

then, when saving the search (Finder - cmd F - [for the rest, see: http://christianboyce.blogspot.de/2011/08/bring-back-for-in-lion-finder-sidebar.html]) set the checkbox "Hide Extension" (this option is not available if "show all extensions" in Finder Preferences is checked).

You will have to recreate your searches since the option to hide the extension is not present if "Show all extensions" is enabled.

Disable "Show all extensions" in Finder preferences (which you have done) Create a new Search in Finder Check the box "Hide Extension" in the resulting dialog

See the original author's explanation here for more information