Add fs arg to mg_file* and mg_http_upload API

src/http.h

@@ -58,7 +58,7 @@ size_t mg_url_encode(const char *s, size_t n, char *buf, size_t len);
 void mg_http_creds(struct mg_http_message *, char *, size_t, char *, size_t);
 bool mg_http_match_uri(const struct mg_http_message *, const char *glob);
 int mg_http_upload(struct mg_connection *, struct mg_http_message *hm,
-                   const char *dir);
+                   struct mg_fs *fs, const char *dir);
 void mg_http_bauth(struct mg_connection *, const char *user, const char *pass);
 struct mg_str mg_http_get_header_var(struct mg_str s, struct mg_str v);
 size_t mg_http_next_multipart(struct mg_str, size_t, struct mg_http_part *);