httpd: add .json media type

esp32/max80/httpd.c

@@ -70,6 +70,7 @@ static const struct mime_type mime_types[] = {
     { ".pdf",   4, "application/pdf"           },
     { ".js",    3, "text/javascript"           },
     { ".mjs",   4, "text/javascript"           },
+    { ".json",  5, "application/json"          },
     { ".xml",   4, "text/xml"                  },
     { ".bin",   4, "application/octet-stream"  },
     { ".fw",    3, "application/octet-stream"  },