MIME stands for "Multipurpose Internet Mail Extensions. It's a way of identifying files on the Internet according to their nature and format. For example, using the "Content-type" header value defined in a HTTP response, the browser can open the file with the proper extension/plugin.
We can disable this option when backend server accepts cross domain SSO. This option is available in 188.8.131.52 firmware and above.This can be enabled on diag page. Below screenshot shows the diag page.