The carton override list must be a JSON file. The following shows the format of a carton override list:

{
  "mediaSizes": [
    {"bid": 804,  "bxid": "804", "length": 20, "width": 20, "height": 10, "icon": "original-icon.svg", "state": false},
    {"bid": 806,  "bxid": "806", "length": 30, "width": 20, "height": 10, "icon": "original-icon.svg", "state": false},
    {"bid": 8,  "bxid": "8", "length": 24, "width": 12, "height": 6, "icon": "original-icon.svg", "state": false},
    {"bid": 9,  "bxid": "9", "length": 18, "width": 18, "height": 6, "icon": "original-icon.svg", "state": false},
    {"bid": 10, "bxid": "10", "length": 30, "width": 12, "height": 6, "icon": "original-icon.svg", "state": false},
    {"bid": 12, "bxid": "12", "length": 24, "width": 18, "height": 6, "icon": "original-icon.svg", "state": false},
  ]
}

Invalid JSON files will cause issues. You can ensure the validity of your JSON file using one of many free JSON validation websites on the Internet.