Fix #2922: file uploads fail to render curl command

The request body is an immutable.js OrderedMap rather than a string,
so this fixes the type error:

  TypeError: request.get(...).split is not a function

Adds a special curl command argument for files, e.g.

  curl -F "param=value" -F "file=@filename.txt;type=text/plain"

src/core/window.js

@@ -3,7 +3,8 @@ function makeWindow() {
     location: {},
     history: {},
     open: () => {},
-    close: () => {}
+    close: () => {},
+    File: function() {}
   }
 
   if(typeof window === "undefined") {