handler.c: Partly fix plural message about attachment size

handler.c: Partly fix plural message about attachment size

Use ngettext() to choose a (more) correct plural form for 'byte'
depending on the attachment size. There is a caveat though: We do not
print the size as number but as a prettified number, e.g.  2048 will be
printed as 2K. Some languages might inflect 'byte' different depending
on whether it is '1K byte' or '1024'. Sadly there is nothing we can do
about that in the moment (besides not using the prettified version but
that is not an option).