I know there are existing codes to build a form that can upload multiple files (bulk import) such as this

Note:

• In this case, the access token is retrieved from Google Apps Script side using ScriptApp.getOAuthToken(). For testing this script, you can use the scope of https://www.googleapis.com/auth/drive. But, if you want to keep the security when you use this script in your actual situation, I would like to propose to use https://www.googleapis.com/auth/drive.file as the scope. By this, the access token can access only the files created by this client.

• When you modified the Google Apps Script, please modify the deployment as a new version. By this, the modified script is reflected in Web Apps. Please be careful this.

• You can see the detail of this in the report of "Redeploying Web Apps without Changing URL of Web Apps for new IDE".

References:

• Web Apps
• Taking advantage of Web Apps with Google Apps Script
• Resumable Upload of Multiple Files with Asynchronous Process for Google Drive