Added upload example

example/upload.cc

@@ -0,0 +1,49 @@
+//
+//  upload.cc
+//
+//  Copyright (c) 2019 Yuji Hirose. All rights reserved.
+//  MIT License
+//
+
+#include <httplib.h>
+#include <iostream>
+#include <fstream>
+using namespace httplib;
+using namespace std;
+
+const char* html = R"(
+<form id="formElem">
+  <input type="file" name="file" accept="image/*">
+  <input type="submit">
+</form>
+<script>
+  formElem.onsubmit = async (e) => {
+    e.preventDefault();
+    let res = await fetch('/post', {
+      method: 'POST',
+      body: new FormData(formElem)
+    });
+    console.log(await res.text());
+  };
+</script>
+)";
+
+int main(void) {
+  Server svr;
+
+  svr.Get("/", [](const Request & /*req*/, Response &res) {
+    res.set_content(html, "text/html");
+  });
+
+  svr.Post("/post", [](const Request & req, Response &res) {
+    auto file = req.get_file_value("file");
+    cout << "file: " << file.offset << ":" << file.length << ":" << file.filename << endl;
+
+    ofstream ofs(file.filename, ios::binary);
+    ofs << req.body.substr(file.offset, file.length);
+
+    res.set_content("done", "text/plain");
+  });
+
+  svr.listen("localhost", 1234);
+}