Remember, I have no a priori knowledge of the input file encoding.
That's the fundamental problem to solve.
If the file could be using any encoding, then there is no concept of reading "line by line" as you can't possibly tell what the line ending is.
I suggest you first address this part, and the rest will be easy. Now, without knowing the context it's hard to say whether that means you should be asking the user for the encoding, or detecting it heuristically, or something else - but I wouldn't start trying to use the data before you can fully understand it.