try content-name if disposition-filename isn't set

I've encountered attachments where content-name is set instead of disposition-filename in the result of `mailparse_msg_get_part_data`, I assume because the part's header only has a filename under the content header and not the disposition header
```
Content-Type: application/octet-stream; name="filename.csv"
Content-Transfer-Encoding: base64
Content-Disposition: attachment
```
whereas an attachment that successfully parses already has the filename under both headers:
```
Content-Type: text/csv; charset=us-ascii; 
        name=filename.csv
Content-Transfer-Encoding: 7bit
Content-Disposition: attachment; 
        filename=filename.csv
```