//https://api.slack.com/methods/files.upload
//https://qiita.com/denzow/items/c85b673d93bb59405a73



const SLACK_TOKEN = 'xoxb-xxx-xxxx';
const SLACK_POST_URL = 'https://slack.com/api/files.upload';
var blob = '';



/*

channel でなく、channels　に
formData を直接 ajaxのdataに
    contentType: false,
    processData: false,
が必要
*/
function submit(){
  var file = new File([blob], 'filename');

  var data = {
      'channels':['xxxx'],
      'text':'test...',
      'filename':'filename',
      'file' : file,
      'title':'title',
      initial_comment: '投稿時のコメント',
  };
  var formData = new FormData();
  for(let key in data){
      formData.append(key, data[key]);
  }
  var dataJson = JSON.stringify(data);
  console.log(dataJson);
  $.ajax({
      headers: {
        contentType: 'multipart/form-data',
        authorization: 'Bearer ' + SLACK_TOKEN
      },
    type: 'POST',
    contentType: 'multipart/form-data',
    url: SLACK_POST_URL,
    data: formData,
    contentType: false,
    processData: false,
    success: function (data) {
      console.log(data);
    },
    error: function(xhr,status,error){              
      console.log(xhr);           
      console.log("error: " + error);
    },
  });
}


//画像取得後に投稿処理を行う
fetch('slack_api_logo.png').then(function(response) {
  if(response.ok) {
    return response.blob();
  }
  throw new Error('Network response was not ok.');
}).then(function(myBlob) {
  blob = myBlob;
  submit();
});

ページのソースを表示 : Ctrl+U , DevTools : F12

view-source:https://hi0a.com/demo/-js/js-slack-api-token-file-upload/