insomnia/packages/insomnia-httpsnippet/test/fixtures/requests/multipart-data.json

{
  "method": "POST",
  "url": "http://mockbin.com/har",
  "headers": [
    {
      "name": "content-type",
      "value": "multipart/form-data"
    }
  ],
  "postData": {
    "mimeType": "multipart/form-data",
    "params": [
      {
        "name": "foo",
        "value": "Hello World",
        "fileName": "hello.txt",
        "contentType": "text/plain"
      }
    ]
  }
}