The ext.json file contains metadata for the extension. For example:

{
  "extensionID": "c2e6a60e-579a-4190-af3e-5edc0cd8a725",
  "developerID": "999999",
  "createdBy": "Demo Corp.",
  "name": "ShopperContextSelectorDemoWidget",
  "version": 1,
  "timeCreated": "2017-10-10",
  "description": "Demo Shopper Context Widget"
}

Note that the extensionID must match the value generated on the Extensions page in the administration interface. See Install the widget for more information.