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Format attachment file names into mime part headers safely (#31)

Previously, the file name went into the Content-Type name parameter by
plain string concatenation, so a name containing CR or LF ended the header
and injected part headers and a body of its own. Content-Disposition was
built with %q and escaped those, but its quoting was still wrong for names
holding a quote, a semicolon or non-ascii characters.

After this change, both headers are built with mime.FormatMediaType, which
quotes and RFC 2231 encodes the name, and a file name containing CR or LF
is rejected outright, which also keeps Content-ID safe.

The generated headers change shape for every attachment: parameters are no
longer quoted when they don't need to be, so tests assert file names from
the parsed mime tree instead of matching the serialized header text.

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