message/rfc822 w/o filename is ignored

An attachment with following headers is not being extracted into a separate file:
```
Content-Type: message/rfc822
Content-Transfer-Encoding: 7bit
Content-Disposition: attachment
```
It works if I add `name` attribute:
```
Content-Type: message/rfc822; name=msg.eml
Content-Transfer-Encoding: 7bit
Content-Disposition: attachment
```

ripmime v1.4.1.0 - September 20, 2022