Not really how it works, but you can send data in just about any format over syslog. On the receiving end the syslog server doesn't know if it should write in txt or png or doc or whatever format you sent though. You have to configure it as such. Splunk as a syslog receiver can't do much with encoded file types though. It's just going to write the 0s and 1s (most likely hexidecimal) that is inside the data file you sent.
The short answer is no, you don't send files as attachments in syslog. Instead you're streaming data through in whatever format you choose and syslog is going to write what it receives, along with a default msg header at the top of the syslog event (timestamp and host name it received the data from).
