Minor fix for adding attachments

There was a [minor error on a previous commit](https://github.com/sendgrid/sendgrid-python/blob/3d3ec5a6b2b9d6ee34048b828ef9d29f1ac23e5c/sendgrid/message.py#L114), where `f` should have  actually been `file`. I've gone ahead and fixed the error, and also appended an underscore to the `file` variable name (since `file` is a built-in Python function, and can be slightly confusing).