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`image`. The message part contains a binary representation of an image. The subtype indicates the format of the image such as `gif <http://en.wikipedia.org/wiki/Graphics_Interchange_Format>`_, `jpg <http://en.wikipedia.org/wiki/Jpeg>`_ or `png <http://en.wikipedia.org/wiki/Portable_Network_Graphics>`_.
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`audio`. The message part contains an audio clip. The subtype indicates the format of the audio clip like `wav <http://en.wikipedia.org/wiki/Wav>`_ or `mp3 <http://en.wikipedia.org/wiki/Mp3>`_.
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`video`. The message part contains a video clip. The subtype indicates the format of the video clip like `avi <http://en.wikipedia.org/wiki/Audio_Video_Interleave>`_ or `mp4 <http://en.wikipedia.org/wiki/Mp4>`_.
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`application`. The message part contains binary information that was produced by the particular application listed as the subtype. Email clients use the subtype to launch the application that is able to decode the received binary information.
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From ASCII to Unicode
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The first computers used different techniques to represent characters in memory and on disk. During the 1960s, computers began to exchange information via tape or telephone lines. Unfortunately, each vendor had its own proprietary character set and exchanging data between computers from different vendors was often difficult. The 7 bits ASCII character table :rfc:`20` was adopted by several vendors and by many Internet protocols. However, ASCII became a problem with the internationalization of the Internet and the desire of more and more users to use character sets that support their own written language. A first attempt at solving this problem was the definition of the ISO-8859_ character sets by :term:`ISO`. This family of standards specified various character sets that allowed the representation of many European written languages by using 8 bits characters. Unfortunately, an 8-bits character set is not sufficient to support some widely used languages, such as those used in Asian countries. Fortunately, at the end of the 1980s, several computer scientists proposed to develop a standard that supports all written languages used on Earth today. The Unicode standard [Unicode]_ has now been adopted by most computer and software vendors. For example, Java always uses Unicode to manipulate characters, Python can handle both ASCII and Unicode characters. Internet applications are slowly moving towards complete support for the Unicode character sets, but moving from ASCII to Unicode is an important change that can have a huge impact on current deployed implementations. See, for example, the work to completely internationalize email :rfc:`4952` and domain names :rfc:`5890`.
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The last MIME header line is `Content-Transfer-Encoding:`. This header line is used after the `Content-Type:` header line, within a message part, and specifies how the message part has been encoded. The default encoding is to use 7 bits ASCII. The most frequent encodings are `quoted-printable` and `Base64`. Both support encoding a sequence of bytes into a set of ASCII lines that can be safely transmitted by email servers. `quoted-printable` is defined in :rfc:`2045`. We briefly describe `base64` which is defined in :rfc:`2045` and :rfc:`4648`.
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`Base64` divides the sequence of bytes to be encoded into groups of three bytes (with the last group possibly being partially filled). Each group of three bytes is then divided into four six-bit fields and each six bit field is encoded as a character from the table below.
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Value
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Encoding
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0
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A
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17
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R
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34
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i
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51
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z
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1
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|
B
|
