In SharePoint, you must allow .json file type for HTML5 output. To allow this file type,
- In Central Administration, navigate to Manage Web Applications.
- Highlight the Web Application you are uploading to.
- Click on Blocked File Types in the ribbon.
- Find the file type, in this case .json, and deselect it from the list, then click OK.
Contact your SharePoint administrator to allow .json files in case the above steps do not work.