Contacts have 12 text extra fields and 4 date extra fields. This endpoint returns the name and type of each one, keyed by the identifier used in a contact's extra_fields:

{
  "extra_fields": {
    "com.marketcircle.daylite/extra1": { "display_name": "Membership", "type": "string" },
    "com.marketcircle.daylite/extra_date_1": { "display_name": "Renewal Date", "type": "datetime" }
  }
}

Fields that have not been renamed have their default names, Extra Field 1 to Extra Field 12 and Extra Date 1 to Extra Date 4.