sendto plugin: fix content type resolution. (Bug #15916)

Additionally request G_FILE_ATTRIBUTE_STANDARD_CONTENT_TYPE for tse_data->info.
This is needed later for g_file_info_get_content_type() call inside tse_file_is_archive()

Signed-off-by: Sergey Ponomarev <stokito@gmail.com>
Signed-off-by: Alexander Schwinn <alexxcons@xfce.org>