msSaveBlob - Web APIs
notes when a site calls this method, the behavior is the same as when windows internet explorer downloads a file with the following in the header, where x-download-options removes the file open button from the browser file download dialog: content-length: <blob.size> content-type: <blob.type> content-disposition: attachment;filename=<defaultname> x-download-options: noopen specifications not part of any specifications.

HTTP headers - HTTP
x-download-options the x-download-options http header indicates that the browser (internet explorer) should not display the option to "open" a file that has been downloaded from an application, to prevent phishing attacks as the file otherwise would gain access to execute in the context of the application.