Can I harvest .txt file from filebeat?

I'm trying to send my system_log to logstash.
But unfortunately my system_log file is text file(.txt) rather than (.log)
For this reason I can't send it to logstash. (It is available when I change my file extension to .log)
I don't want to convert .txt to .log. manually.
is there any method to solve this problem??

Why? Logstash doesn't care what extension your files are.