How a file can know what type of data save?

Question

And how can I do that?
Can I from a database?

Answer 1

I assume you want to figure out what is stored in a random file. The ending can give you hints, you can do other types of analysis, but you can't know surely.
You can imagine a file as a vector<char> on your hard drive.

Answer 2

I am in dark room, I can't see anything