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/components/curl.jsx

@@ -19,7 +19,7 @@ export default class Curl extends React.Component {
       <div>
         <h4>Curl</h4>
         <div className="copy-paste">
-          <textarea onFocus={this.handleFocus} className="curl" style={{ whiteSpace: "normal" }} value={curl}></textarea>
+          <textarea onFocus={this.handleFocus} readOnly="true" className="curl" style={{ whiteSpace: "normal" }} value={curl}></textarea>
         </div>
       </div>
     )