Make a Lightning Button Download a .html Report

The Attachment object is for saving data to the database. It has a number of fields, which is why you're seeing [object Object]. Instead, you should be able to do so with code like this:

@AuraEnabled public static String getFile() {
  String htmlText = '<html>...</html>';
  return EncodingUtil.base64Encode(Blob.valueOf(htmlText));

action.setCallback(this, function(response) {
       let state = response.getState();
       if (state === "SUCCESS") {
           let downloadLink = document.createElement("a");
           downloadLink.href = "data:text/html;base64,"+response.getReturnValue();
  = "filename.html";
       } ...