ASCII
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... 822std11(-> 2822prop) headers they often appear in,
are limited to 7bit US-ASCII, and the encoded-word mechanisms
of RFC 2047draft are not available to parameter values ...
... parameter values in character sets other
than US-ASCII without specifying some sort of private per-
parameter encoding.
...
... attribute := 1*attribute-char
attribute-char := <any (US-ASCII) CHAR except SPACE, CTLs,
"*", "'", "%", or tspecials>
...
... Moore, K., "Multipurpose Internet Mail Extensions (MIME) Part Three: Representation of Non-ASCII Text in Internet Message Headers", RFC 2047draft ...