_mongoc_gridfs_file_extend has at least two clearly incorrect statements. First:

      /* Set bytes until we reach the limit or fill a page */
      file->pos += _mongoc_gridfs_file_page_memset0 (file->page,
                                                     target_length - file->pos);

This is wrong because the return type of _mongoc_gridfs_file_page_memset0 is bool, and it is being added to a uint64_t. (Introduced in e2c174.) Second:

   BSON_ASSERT (file->length = target_length);

This appears wrong because it is an assignment in an assert (CDRIVER-1896), but it is not compiled out in a release build any more (CDRIVER-697). The intention is mysterious.