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MIME Content-Transfer-Encoding Header Field

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Explains the Content-Transfer-Encoding header field and how it affects how data is stored in the MIME.

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Local $bSuccess = False

;  The Content-Transfer-Encoding header field is typically set to one of these values:
;  
;    base64
;    quoted-printable
;    binary
;    8bit
;    7bit

;  If the encoding is base64 or quoted-printable, then the bytes of the content are encoded as such.
;  The values "8bit", "7bit", and "binary" all imply that NO encoding has been performed.

;  Here are some guidelines and hints:
;  
;  1) "8bit", "7bit", and "binary" encodings (i.e. NO encoding) results in the smallest MIME.

;  2) A "binary" encoding implies non-text data.  Binary MIME CANNOT be retrieved as a string.  It can only be retrieved as a byte array.

;  3) "8bit" and "7bit" encodings imply text.  
;     7bit implies that the text is us-ascii (all byte values less than or equal to 0x7F).
;     8bit implies that the text contains non-us-ascii chars.

;  4) Base64 is the best choice for encoding non-text data, such as PDF's, images, etc.

;  5) quoted-printable is the best choice for encoding text data where most chars are going to be us-ascii. This would typically 
;     include all Western European languages.  For Asian, Arabic, Hebrew, etc. where most chars in the text are non-us-ascii,
;     the most efficient encoding would be base64.  

;  Let's demonstrate with this small JPG image.

$oSbJpgBase64 = ObjCreate("Chilkat.StringBuilder")
$oSbJpgBase64.Append("/9j/4AAQSkZJRgABAQEASABIAAD//gAmRmlsZSB3cml0dGVuIGJ5IEFkb2JlIFBob3Rvc2hvcD8g" & @CRLF)
$oSbJpgBase64.Append("NC4w/9sAQwAQCwwODAoQDg0OEhEQExgoGhgWFhgxIyUdKDozPTw5Mzg3QEhcTkBEV0U3OFBtUVdf" & @CRLF)
$oSbJpgBase64.Append("YmdoZz5NcXlwZHhcZWdj/9sAQwEREhIYFRgvGhovY0I4QmNjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj" & @CRLF)
$oSbJpgBase64.Append("Y2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj/8IAEQgAFAAUAwERAAIRAQMRAf/EABcAAAMBAAAA" & @CRLF)
$oSbJpgBase64.Append("AAAAAAAAAAAAAAIDBAX/xAAYAQADAQEAAAAAAAAAAAAAAAABAgMEAP/aAAwDAQACEAMQAAAB2kZY" & @CRLF)
$oSbJpgBase64.Append("NNEijWKddfTmLgALWH//xAAbEAACAgMBAAAAAAAAAAAAAAABAgMRAAQSE//aAAgBAQABBQL0XqN+" & @CRLF)
$oSbJpgBase64.Append("pM2aqJGMiqFFCyg7z//EABwRAAICAgMAAAAAAAAAAAAAAAERAAIQIQMSUf/aAAgBAwEBPwHqU5aq" & @CRLF)
$oSbJpgBase64.Append("Axx+y1tMQl4elj//xAAcEQEAAQUBAQAAAAAAAAAAAAABEQACEBIhA1H/2gAIAQIBAT8B3Bhqy7Zc" & @CRLF)
$oSbJpgBase64.Append("enyiwmGgDhiOzj//xAAdEAABAwUBAAAAAAAAAAAAAAABAAIREBIhIkFR/9oACAEBAAY/ArZyn+Cg" & @CRLF)
$oSbJpgBase64.Append("xtxWuJaoCnqDuin/xAAcEAABBAMBAAAAAAAAAAAAAAABABEhYRAxQVH/2gAIAQEAAT8hkEwPUUR9" & @CRLF)
$oSbJpgBase64.Append("DYfE4nxtRpIkBTsayuALIiuY/9oADAMBAAIAAwAAABDWPTsf/8QAGhEAAwADAQAAAAAAAAAAAAAA" & @CRLF)
$oSbJpgBase64.Append("AAEREDFBIf/aAAgBAwEBPxC0DVPcWm+Ce4OesrkE6bjH/8QAGBEBAQEBAQAAAAAAAAAAAAAAAREA" & @CRLF)
$oSbJpgBase64.Append("QRD/2gAIAQIBAT8QahMiOc8YgSrnTY3ELclHXn//xAAcEAEBAAIDAQEAAAAAAAAAAAABEQAhMUFx" & @CRLF)
$oSbJpgBase64.Append("EFH/2gAIAQEAAT8Qn3igmSZSj+c4N4zapMy9IjFV98wncN2iuLFsCEbDGxQkI6RO/n//2Q==" & @CRLF)

$oMime = ObjCreate("Chilkat.Mime")
$oMime.ContentType = "image/jpeg"
$oMime.SetBodyFromEncoded("base64",$oSbJpgBase64.GetAsString())

ConsoleWrite($oMime.GetMime() & @CRLF)
ConsoleWrite("-" & @CRLF)

;  The result:
;  
;  	Content-Type: image/jpeg
;  	Content-Transfer-Encoding: base64
;  
;  	/9j/4AAQSkZJRgABAQEASABIAAD//gAmRmlsZSB3cml0dGVuIGJ5IEFkb2JlIFBob3Rvc2hvcD8g
;  	NC4w/9sAQwAQCwwODAoQDg0OEhEQExgoGhgWFhgxIyUdKDozPTw5Mzg3QEhcTkBEV0U3OFBtUVdf
;  	YmdoZz5NcXlwZHhcZWdj/9sAQwEREhIYFRgvGhovY0I4QmNjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj
;  	Y2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj/8IAEQgAFAAUAwERAAIRAQMRAf/EABcAAAMBAAAA
;  	AAAAAAAAAAAAAAIDBAX/xAAYAQADAQEAAAAAAAAAAAAAAAABAgMEAP/aAAwDAQACEAMQAAAB2kZY
;  	NNEijWKddfTmLgALWH//xAAbEAACAgMBAAAAAAAAAAAAAAABAgMRAAQSE//aAAgBAQABBQL0XqN+
;  	pM2aqJGMiqFFCyg7z//EABwRAAICAgMAAAAAAAAAAAAAAAERAAIQIQMSUf/aAAgBAwEBPwHqU5aq
;  	Axx+y1tMQl4elj//xAAcEQEAAQUBAQAAAAAAAAAAAAABEQACEBIhA1H/2gAIAQIBAT8B3Bhqy7Zc
;  	enyiwmGgDhiOzj//xAAdEAABAwUBAAAAAAAAAAAAAAABAAIREBIhIkFR/9oACAEBAAY/ArZyn+Cg
;  	xtxWuJaoCnqDuin/xAAcEAABBAMBAAAAAAAAAAAAAAABABEhYRAxQVH/2gAIAQEAAT8hkEwPUUR9
;  	DYfE4nxtRpIkBTsayuALIiuY/9oADAMBAAIAAwAAABDWPTsf/8QAGhEAAwADAQAAAAAAAAAAAAAA
;  	AAEREDFBIf/aAAgBAwEBPxC0DVPcWm+Ce4OesrkE6bjH/8QAGBEBAQEBAQAAAAAAAAAAAAAAAREA
;  	QRD/2gAIAQIBAT8QahMiOc8YgSrnTY3ELclHXn//xAAcEAEBAAIDAQEAAAAAAAAAAAABEQAhMUFx
;  	EFH/2gAIAQEAAT8Qn3igmSZSj+c4N4zapMy9IjFV98wncN2iuLFsCEbDGxQkI6RO/n//2Q==
;  

;  Set the Content-Transfer-Encoding to "binary" by setting the Encoding property.
$oMime.Encoding = "binary"

;  Try to get the MIME string.  This CANNOT be done.  The bytes of the JPG image do not
;  represent chars, and to return a string means that bytes must be interpreted according
;  to some character encoding (such as utf-8).  Non-text binary bytes can only be 
;  contained in a string IF encoded in some way. Encodings such as Base64, quoted-printable,
;  URL, etc. exist to make it possible to represent binary data in string format.   
ConsoleWrite($oMime.GetMime() & @CRLF)
ConsoleWrite("-" & @CRLF)

;  We CAN get the binary MIME as bytes..
Local $oMimeBytes
$oMimeBytes = $oMime.GetMimeBytes()

;  Regardless of the Content-Transfer-Encoding, the 
;  body content can always be retrieved and the body bytes
;  decoded from whatever encoding is used..
Local $oJpgBytes
$oJpgBytes = $oMime.GetBodyBinary()

;  To get the body in base64 format, first make sure
;  the Content-Transfer-Encoding is base64, then call GetBodyEncoded.
$oMime.Encoding = "base64"
Local $sJpgBase64 = $oMime.GetBodyEncoded()
ConsoleWrite($sJpgBase64 & @CRLF)
ConsoleWrite("-" & @CRLF)

;  Let's go back to "binary" MIME..
$oMime.Encoding = "binary"

;  Let's say we have MIME, and it was loaded directly from a file, or from
;  a byte array.  (It was not loaded from the contents of a string variable.)
;  We don't know whether the MIME contains binary or 8bit encodings, and thus
;  we dont' know if the MIME is safe to get as a string.
;  The Convert8Bit method can be called to recursively traverse the MIME and set
;  all 8bit or binary encodings to "base64".  This makes the MIME safe for storing in
;  a string.
$oMime.Convert8Bit 
Local $sMimeStr = $oMime.GetMime()
ConsoleWrite($sMimeStr & @CRLF)
ConsoleWrite("-" & @CRLF)