These changes were already part of bmclib v1 but missed out from being included in v2,
#279

…t instead

This makes the code easier to read and maintain.

As of now we limit the method to expect an os.File as an io.Reader,
once its clear theres other implementations of io.Readers required we
can accept those and figure how to determine the file size for the
multipart payload

This makes the UpdateParameters handling easier to grok and maintain