A year or two ago when I first read about it, I saw in the software faq, or maybe it was a post by its creator on HN itself or reddit, that they will make it paid once it reaches stable version(version 1?).
what do people think about installing new software from less well known companies and giving the app full access to all your files? how would i convince myself that this software won't upload my data in the background?
The author here. Marta doesn't perform any network activity besides update checks, which you can disable. If you wish, you can block all network access for the application in your firewall.
I personally have a hard time trusting closed source software random sources I've never heard of before. Maybe I'm just too paranoid but I try not to run random software especially something that has as widespread access to my data as a file-manager.