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/utils.js

@@ -7,6 +7,7 @@ import _memoize from "lodash/memoize"
 import some from "lodash/some"
 import eq from "lodash/eq"
 import { memoizedSampleFromSchema, memoizedCreateXMLExample } from "core/plugins/samples/fn"
+import win from "./window"
 
 const DEFAULT_REPONSE_KEY = "default"
 
@@ -34,6 +35,9 @@ export function fromJSOrdered (js) {
   if(isImmutable(js))
     return js // Can't do much here
 
+  if (js instanceof win.File)
+    return js
+
   return !isObject(js) ? js :
     Array.isArray(js) ?
       Im.Seq(js).map(fromJSOrdered).toList() :