From Xojo Documentation
This is the MIME type of the content. Browsers rely on this to determine what to do with the content. Setting WebFile.ForceDownload to True will override the MIMEType. Defaults to "application/octet-stream".
This example creates a text file to make available for download. You can put it on the Action event handler for a WebButton so that the file is downloaded when the user clicks the button:
TextFile.MimeType = "text/plain"
TextFile.ForceDownload = True // If False, the browser may try to display the file instead of download it
TextFile.FileName = "TextFile.txt"
TextFile.Data = "Hello, world!"
ShowURL(TextFile.URL) // This causes the file to be downloaded