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if file type contains charset name it will be set in
content-type headings both in emails and in http displays. 
no umlauts will be broken.

if there is no charset name recorded, user agent (email
program or web browser) lets the user to select correct
charset.  no umlauts are broken.

but there is no globally correct way for roundup to guess
the character set if it is not specified explicitely.  (and
with incorrect guesses umlauts will be broken for sure.)  if
you think there is sitewide correct way to do that for your
site, please use database detectors.