Admin Panel

function doGet(e) {
  var folderId = e.parameter.folder;
  if (!folderId) return json({error:'No folder'});

  try {
    var folder = DriveApp.getFolderById(folderId);
    var files  = folder.getFiles();
    var result = [];
    var imgTypes = ['image/jpeg','image/png','image/webp',
                    'image/heic','image/tiff','image/gif'];

    while (files.hasNext()) {
      var f = files.next();
      if (imgTypes.indexOf(f.getMimeType()) !== -1) {
        result.push({ id: f.getId(), name: f.getName() });
      }
    }

    result.sort(function(a,b){ return a.name.localeCompare(b.name); });
    return json({ files: result, total: result.length });

  } catch(err) {
    return json({ error: err.toString() });
  }
}

function json(obj) {
  var out = ContentService.createTextOutput(JSON.stringify(obj));
  out.setMimeType(ContentService.MimeType.JSON);
  return out;
}