Am I rational to fear that my idea can be stolen?

I am in the process of writing a story. I have written the outline and even the first chapter. Because I work with someone from far away for proofreading and other help, I posted my outline and the chapter on Mediafire and sent the link to them. I didn’t set any password for the files.

A while ago I finished my second chapter. I was about to post it on Mediafire again when I found that my outline and chapter 1 had been downloaded six times. That led me to wonder: is there anyone apart from that person who can download the files?

And if there are other people who can get access to the files, I’m afraid that they can take what I wrote and claim it to be theirs. It literally means my work is stolen. Can something like that happen? Is my fear rational?