It was invalid JSON because of the last added , There might be a shorter way of writing in Mockoon (?? would love to hear) but this generates valid JSON, using handlebarsjs if @last and else

    "thumbnailItemIds": [
  {{#each @firstItems}}
    "{{id}}"
    {{#if @last}}
    {{else}}
    ,
    {{/if}}
  {{/each}}
]