mod_http_file_share: Add media-src 'self' to Content-Security-Policy header

This allows certain media files to be loaded when navigated to directly in a
web browser.

Note that in some browsers (Chrome), the media gets transformed
internally into a HTML page with some basic styles, but these are blocked due
to our default-src policy of 'none' Although this could be unblocked with
style-src unsafe-inline, it is not our plan to fix this, because this would
have negative security implications.

The reason for our CSP is to prevent the file share service from being used to
host malicious HTML/CSS/JS. Yes, CSS can be malicious.

Our file share service is for uploading and downloading files, it is not a
substitute for website/content hosting.

plugins/mod_http_file_share.lua

@@ -469,7 +469,7 @@ function handle_download(event, path) -- GET /uploads/:slot+filename
 	response.headers.accept_ranges = "bytes";
 
 	response.headers.cache_control = "max-age=31556952, immutable";
-	response.headers.content_security_policy =  "default-src 'none'; frame-ancestors 'none';"
+	response.headers.content_security_policy =  "default-src 'none'; media-src 'self'; frame-ancestors 'none';"
 	response.headers.strict_transport_security = "max-age=31556952";
 	response.headers.x_content_type_options = "nosniff";
 	response.headers.x_frame_options = "DENY"; -- COMPAT IE missing support for CSP frame-ancestors