HTML form file-input fields are not converted to compatible blob format when submitted. They are left in application/octet-stream format. The file-input fields are passed as application/octet-stream, so DriveApp.createFile(formBlob) does not work correctly … So the functionality provided by HtmlService is not consistent with the documentation
There was an annoying little Google Apps Script issue which meant when handling file uploads to published Web Apps some additional coding was required rather than just sending the file blob server side. Thankfully the open issue ticket for this has recently been marked and confirmed fixed.