(Visual FoxPro) MIME Content-Transfer-Encoding Header Field

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

Chilkat ActiveX Downloads ActiveX for 32-bit and 64-bit Windows

LOCAL lnSuccess
LOCAL loSbJpgBase64
LOCAL loMime
LOCAL loMimeBytes
LOCAL loJpgBytes
LOCAL lcJpgBase64
LOCAL lcMimeStr

* 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.

* For versions of Chilkat < 10.0.0, use CreateObject('Chilkat_9_5_0.StringBuilder')
loSbJpgBase64 = CreateObject('Chilkat.StringBuilder')
loSbJpgBase64.Append("/9j/4AAQSkZJRgABAQEASABIAAD//gAmRmlsZSB3cml0dGVuIGJ5IEFkb2JlIFBob3Rvc2hvcD8g" + CHR(13) + CHR(10))
loSbJpgBase64.Append("NC4w/9sAQwAQCwwODAoQDg0OEhEQExgoGhgWFhgxIyUdKDozPTw5Mzg3QEhcTkBEV0U3OFBtUVdf" + CHR(13) + CHR(10))
loSbJpgBase64.Append("YmdoZz5NcXlwZHhcZWdj/9sAQwEREhIYFRgvGhovY0I4QmNjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj" + CHR(13) + CHR(10))
loSbJpgBase64.Append("Y2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj/8IAEQgAFAAUAwERAAIRAQMRAf/EABcAAAMBAAAA" + CHR(13) + CHR(10))
loSbJpgBase64.Append("AAAAAAAAAAAAAAIDBAX/xAAYAQADAQEAAAAAAAAAAAAAAAABAgMEAP/aAAwDAQACEAMQAAAB2kZY" + CHR(13) + CHR(10))
loSbJpgBase64.Append("NNEijWKddfTmLgALWH//xAAbEAACAgMBAAAAAAAAAAAAAAABAgMRAAQSE//aAAgBAQABBQL0XqN+" + CHR(13) + CHR(10))
loSbJpgBase64.Append("pM2aqJGMiqFFCyg7z//EABwRAAICAgMAAAAAAAAAAAAAAAERAAIQIQMSUf/aAAgBAwEBPwHqU5aq" + CHR(13) + CHR(10))
loSbJpgBase64.Append("Axx+y1tMQl4elj//xAAcEQEAAQUBAQAAAAAAAAAAAAABEQACEBIhA1H/2gAIAQIBAT8B3Bhqy7Zc" + CHR(13) + CHR(10))
loSbJpgBase64.Append("enyiwmGgDhiOzj//xAAdEAABAwUBAAAAAAAAAAAAAAABAAIREBIhIkFR/9oACAEBAAY/ArZyn+Cg" + CHR(13) + CHR(10))
loSbJpgBase64.Append("xtxWuJaoCnqDuin/xAAcEAABBAMBAAAAAAAAAAAAAAABABEhYRAxQVH/2gAIAQEAAT8hkEwPUUR9" + CHR(13) + CHR(10))
loSbJpgBase64.Append("DYfE4nxtRpIkBTsayuALIiuY/9oADAMBAAIAAwAAABDWPTsf/8QAGhEAAwADAQAAAAAAAAAAAAAA" + CHR(13) + CHR(10))
loSbJpgBase64.Append("AAEREDFBIf/aAAgBAwEBPxC0DVPcWm+Ce4OesrkE6bjH/8QAGBEBAQEBAQAAAAAAAAAAAAAAAREA" + CHR(13) + CHR(10))
loSbJpgBase64.Append("QRD/2gAIAQIBAT8QahMiOc8YgSrnTY3ELclHXn//xAAcEAEBAAIDAQEAAAAAAAAAAAABEQAhMUFx" + CHR(13) + CHR(10))
loSbJpgBase64.Append("EFH/2gAIAQEAAT8Qn3igmSZSj+c4N4zapMy9IjFV98wncN2iuLFsCEbDGxQkI6RO/n//2Q==" + CHR(13) + CHR(10))

* For versions of Chilkat < 10.0.0, use CreateObject('Chilkat_9_5_0.Mime')
loMime = CreateObject('Chilkat.Mime')
loMime.ContentType = "image/jpeg"
loMime.SetBodyFromEncoded("base64",loSbJpgBase64.GetAsString())

? loMime.GetMime()
? "-"

* 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.
loMime.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.   
? loMime.GetMime()
? "-"

* We CAN get the binary MIME as bytes..

loMimeBytes = loMime.GetMimeBytes()

* Regardless of the Content-Transfer-Encoding, the 
* body content can always be retrieved and the body bytes
* decoded from whatever encoding is used..

loJpgBytes = loMime.GetBodyBinary()

* To get the body in base64 format, first make sure
* the Content-Transfer-Encoding is base64, then call GetBodyEncoded.
loMime.Encoding = "base64"
lcJpgBase64 = loMime.GetBodyEncoded()
? lcJpgBase64
? "-"

* Let's go back to "binary" MIME..
loMime.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.
loMime.Convert8Bit()
lcMimeStr = loMime.GetMime()
? lcMimeStr
? "-"

RELEASE loSbJpgBase64
RELEASE loMime