Base64 is described in RFC1521.
If less than one-fourth of the data that was scanned has the high bit set or if the type in the
Content-Type: header is listed in $=q (?§22.6.10 on page 874), the data is converted from 8 to
7 bits by using quoted-printable encoding, and the following MIME header is inserted into
the stream:
Content-Transfer-Encoding: quoted-printable
Table 24-19. EightBitMode option characters
Key ?§ Meaning
mimify ?§24.9.45.1 on page 1027 Do any necessary conversion of BITMIME to 7-bit.
pass ?§24.9.45.2 on page 1027 Pass unlabeled 8-bit input through as is.
strict ?§24.9.45.3 on page 1027 Reject unlabeled 8-bit input.
* A boundary is used only for multipart messages.
?? Also see the $=q class (?§22.6.10 on page 874) for a way to require quoted-printable encoding.
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24.9 Alphabetized Options | 1027
Under quoted-printable encoding, ASCII control characters (in the range 0x00 through
0x20), the tab character, the = character, and all characters with the high bit set are
converted. First an = character is output, then the character is converted to an ASCII representation
of its hexadecimal value, and that value is output. For example:
0xb9 becomes ?†’ =B9
Under this scheme, the = character is considered binary and is encoded as =3D.
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