I have idea for the file sharing site. Now, I was thinking to first release the service without pricing options and users base, basically, to offer free but limited service, kind a like free sample, and to see how that goes. Than later make the service commercial if people show interest. (to save money on the start) But now I have doubts. Is this really a minimum viable product if it doesn't have product price? How can I test the market when market constitutes paying users? Can you give me some advice on this field?
You seems to be on the good way creating a basic app and see how your users will use it. The feedback you'll get (through analytics, servers logs, and even direct user feedback with tools such as uservoice) will be priceless for your product, because you'll be able to improve it the most possible.
Once your product is really great, start thinking about selling it.
By the way (but this was not the question), be aware that there are plenty of such services and your one will require a killer feature in order to gain users.
If the service you launch with is worth paying for, why wouldn't you start asking money for it from the start?
If the service you launch with isn't worth paying for, why would you launch it?
I know that second question sounds snarky - but it's not meant that way. It can be useful to launch a free option - but you need to know why you're doing it.
The reasons you've given sound pretty fuzzy to me "see how it goes". See how what goes? What numbers are you looking for? How are free users going to tell you useful things about paying users (different markets)? Etc.
You've not really described your business model in detail - but how, for example, do you plan to transition from a free-model to a paying-model?