Modified StringExtension.FormatSize to accept 1024 or 1000 as the base for file size.

Per Vidal information, I modified StringExtension.FormatSize to use either 1024 or 1000 as the base to compute file sizes. I kept backward compatibility with a size of 1024 instead of Apple's 1000 bytes.

Signed-off-by: Stéphane Mons <st.mons.lists@free.fr>