Mint extract is just the essential oils of the mint plant that have been extracted using alcohol. Alcohol is not strictly Paleo, but extracts are used in such small amounts that the alcohol content is negligible. When studies experiment with the health effects of herbs, they usually use the extracts or essential oils of the herbs to get them in their most concentrated form.
The Health Benefits
Mint (and its extract) has proven effective against digestive issues, such as IBS, and has been used to treat cold and flu symptoms, as well as menstrual cramps and headaches.
Mark Sisson even lists mint as one of the healthiest herbs you can include in your diet.