Decode Base64 Email with Apps Script

You can decode base64 encoded email messages in Gmail, or files in your Google Drive, using the base64Decode() method of Google Apps Script.

function base64DecodeEmail() {
  
  // If the base6f.txt file in Drive needs to be decoded
  var mime = DriveApp.getFilesByName("base64.txt").next().getBlob().getDataAsString();
  
  // If a Gmail message with subject base64 needs to be decoded
  var message = GmailApp.search("subject:base64")[0].getMessages()[0];
  var mime = message.getRawContent();

  // Concatenate the chunks in the source
  mime = mime.replace(/\n|\r/g, "");
  
  // Extract the base64 string using regex
  var matches = mime.match(/base64[a-zA-Z0-9\+\/=]+/g);
  
  for (var line in matches) {
    var base64 = matches[line].replace(/^base64/, "");
    var decoded = Utilities.base64Decode(base64);
    var html = Utilities.newBlob(decoded).getDataAsString();
    Logger.log(html);
  }
}