Allow user specify file in mg_http_upload

src/fs.c

@@ -1,5 +1,6 @@
 #include "fs.h"
 #include "printf.h"
+#include "str.h"
 
 struct mg_fd *mg_fs_open(struct mg_fs *fs, const char *path, int flags) {
   struct mg_fd *fd = (struct mg_fd *) calloc(1, sizeof(*fd));