Show file size depend on the OS

[Gao-Jun]

One day, a Windows user asked, why I got a smaller file downloaded (806K) compared with the size shown in 'Files'(825K) ? Is the file downloaded completely?

In Mac OS, 1 KB = 1000 Bytes. In Windows and Linux, 1 MB = 1024 Bytes

This patch will display the file size depend on the OS.

In detail, 1 MB = 1000 Bytes for Mac user, and 1MiB = 1024 Bytes for others