tidy display of file sizes and associated code
further adjust size prefixes

ideas from @manics
fix IEC prefix capitalization
re-use size formatter
switch to traditional SI / JEDEC -like prefixes
adjust copyright headers to current year
use available standard library function instead
fix comment consistency
resurrect code with decimal-point file sizes
fix size of mebibyte
Merge pull request #1446 from mtbc/attachment-size

Improve file size pretty-printing code