Change mg_http_upload API

docs/README.md

@@ -1337,6 +1337,51 @@ A diagram below shows how `mg_http_next_multipart()` in action:
 
 <img src="images/mg_http_next_multipart.svg" alt="Function mg_http_next_multipart()" />
 
+### mg\_http\_upload()
+
+```c
+
+int mg_http_upload(struct mg_connection *c, struct mg_http_message *hm,
+                   struct mg_fs *fs, const char *path, size_t max_size);
+```
+
+This is a helper utility function that is used to upload large files by small
+chunks.
+
+Append HTTP POST data to a file in a specified directory.  A file name and
+file offset are specified by the query string parameters:
+`POST /upload?name=firmware.bin&offset=2048 HTTP/1.1`. If the offset is 0, then the
+file is truncated. It is a client's responsibility to divide a file into a
+smaller chunks and send a sequence of POST requests that will be handled by
+this function.
+
+Parameters:
+- `c`- a connection
+- `hm` - a parsed HTTP message
+- `fs` - a filesystem where to write a file, e.g. `&mg_fs_posix`
+- `path` - a filename
+- `max_size` - a maximum allowed file size
+
+Return value: file size after write, or negative number on error
+
+Usage example:
+
+```c
+static void fn(struct mg_connection *c, int ev, void *ev_data, void *fn_data) {
+  if (ev == MG_EV_HTTP_MSG) {
+    struct mg_http_message *hm = (struct mg_http_message *) ev_data;
+    if (mg_http_match_uri(hm, "/upload")) {
+      mg_http_upload(c, hm, &mg_fs_posix, "/tmp/myfile.bin", 99999);
+    } else {
+      struct mg_http_serve_opts opts = {.root_dir = "."};   // Serve
+      mg_http_serve_dir(c, ev_data, &opts);                 // static content
+    }
+  }
+}
+
+```
+
+
 ## Websocket
 
 ### struct mg\_ws\_message