Instead of loading the entire file into the memory, use the iterator
object natively provided by the file object returned by the open
function.

- This saves memory.
- Avoid __getitem__ call, improve performance.

The inner loop used `range(len(imglist))`, this iterates over the entire
length twice. Using `enumerate` saves one complete iteration.

Furthermore, inside the inner loop `__getitem__` and `__setitem__` were
being called twice in each iteration. Reduces `__getitem__` calls to 0,
as it is already being taken care by for loop, and reduced `__setitem__`
to 1.