convert2Spreadsheet.js

var KEY = 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'; //developer key , get on https://code.google.com/apis/console/  and set enable the Drive API.

/**
 * Convert from Excel data to Spreadsheet
 * @param {Blob} excelFile the excel blob data.
 * @param {String} filename file name on uploading drive
 * @return {Spreadsheet} spreadsheet instance.
 **/
function convert2Spreadsheet(excelFile, filename) {
  var oauthConfig = UrlFetchApp.addOAuthService('drive');
  
  //Create oauth config for drive api
  var scope = 'https://www.googleapis.com/auth/drive';
  oauthConfig.setConsumerKey('anonymous');
  oauthConfig.setConsumerSecret('anonymous');
  oauthConfig.setRequestTokenUrl('https://www.google.com/accounts/OAuthGetRequestToken?scope='+scope);
  oauthConfig.setAuthorizationUrl('https://accounts.google.com/OAuthAuthorizeToken');    
  oauthConfig.setAccessTokenUrl('https://www.google.com/accounts/OAuthGetAccessToken');  
  
  var uploadParams = {
    method:'post',
    oAuthServiceName: 'drive',
    oAuthUseToken: 'always',
    contentType: 'application/vnd.ms-excel',
    contentLength: excelFile.getBytes().length,
    payload: excelFile.getBytes()
  };
  
  //insert to root folder of google drive, and convert to Sheets
  var uploadResponse = UrlFetchApp.fetch('https://www.googleapis.com/upload/drive/v2/files/?uploadType=media&convert=true&key='+KEY, uploadParams);
    
  //this response body format is json , and it has file id. Please see https://developers.google.com/drive/v2/reference/files#resource
  var fileDataResponse = JSON.parse(uploadResponse.getContentText());

  //Update filename
  var updateParams = {
    method:'put',
    oAuthServiceName: 'drive',
    oAuthUseToken: 'always',
    contentType: 'application/json',
    payload: JSON.stringify({ title: filename })
  };
  
  UrlFetchApp.fetch('https://www.googleapis.com/drive/v2/files/'+fileDataResponse.id+'?key='+KEY, updateParams);
  
  return SpreadsheetApp.openById(fileDataResponse.id);
}

sample.js

//Get excel file from site and convert 2 spreadsheet.
var siteUrl = "set your google sites page url";
function convertSiteAttachedExcelFile2Spreadsheet() {
  
  var sitePage = SitesApp.getPageByUrl(siteUrl);
  
  //Get attachments from your page.
  var attachments = sitePage.getAttachments();
  
  if(!attachments || attachments.length == 0) {
    throw new Error("not found attached files.");
  }
  
  //Filter for attachment files.
  var excelFiles = attachments.filter(function(attachment) {
    //var attachment = attachments[0];
    
    return /.+\.xls$/.test(attachment.getTitle())
  });
  
  //var excelFiles = [];
  
  var convertedFiles = [];
  
  //Convert all excel files to spreadsheet.
  for(var i = 0; i < excelFiles.length; i++)
    var excelFile = excelFiles[i];
    Logger.log("converting %s  ...", excelFile.getTitle());
    convertedFiles.push(convert2Spreadsheet(excelFile.getBlob(), excelFile.getTitle()));
  });
  
  
  Logger.log("Done converting.");
  
  
}