Please ask your webservers admin if
<!--#exec cgi="/absolute/filesystem/path/to/script" -->
and
<!--#exec cmd="/absolute/filesystem/path/to/cmd" -->
are allowed in SSI. Some providers disable their interpolation or execution due to security concerns.

If so, there is as it's been already said only the chance of using
<!--#include virtual="/path/relative/to/your/web_server_root/script" -->
which is using the GET method so you cannot use POST but can (using SSI 2.0) add the query_string to it alike this

<!--#include virtual="/path/relative/to/your/web_server_root/script$QU
+ERY_STRING" -->
[download]

Have a nice day
All decision is left to your taste