splitting out Unirest\File and updating README

src/Unirest/File.php

@@ -0,0 +1,19 @@
+<?php
+
+namespace Unirest;
+
+class File
+{
+    /**
+     * Prepares a file for upload. To be used inside the parameters declaration for a request.
+     * @param string $path The file path
+     */
+    public static function add($filename, $mimetype = '', $postname = '')
+    {
+        if (function_exists('curl_file_create')) {
+            return curl_file_create($filename, $mimetype = '', $postname = '');
+        } else {
+            return sprintf('@%s;filename=%s;type=%s', $filename, $postname ?: basename($filename), $mimetype);
+        }
+    }
+}