(PowerShell) MIME Content-Transfer-Encoding Header Field

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

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Add-Type -Path "C:\chilkat\ChilkatDotNet47-9.5.0-x64\ChilkatDotNet47.dll"

# 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.

$sbJpgBase64 = New-Object Chilkat.StringBuilder
$sbJpgBase64.Append("/9j/4AAQSkZJRgABAQEASABIAAD//gAmRmlsZSB3cml0dGVuIGJ5IEFkb2JlIFBob3Rvc2hvcD8g`r`n")
$sbJpgBase64.Append("NC4w/9sAQwAQCwwODAoQDg0OEhEQExgoGhgWFhgxIyUdKDozPTw5Mzg3QEhcTkBEV0U3OFBtUVdf`r`n")
$sbJpgBase64.Append("YmdoZz5NcXlwZHhcZWdj/9sAQwEREhIYFRgvGhovY0I4QmNjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj`r`n")
$sbJpgBase64.Append("Y2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj/8IAEQgAFAAUAwERAAIRAQMRAf/EABcAAAMBAAAA`r`n")
$sbJpgBase64.Append("AAAAAAAAAAAAAAIDBAX/xAAYAQADAQEAAAAAAAAAAAAAAAABAgMEAP/aAAwDAQACEAMQAAAB2kZY`r`n")
$sbJpgBase64.Append("NNEijWKddfTmLgALWH//xAAbEAACAgMBAAAAAAAAAAAAAAABAgMRAAQSE//aAAgBAQABBQL0XqN+`r`n")
$sbJpgBase64.Append("pM2aqJGMiqFFCyg7z//EABwRAAICAgMAAAAAAAAAAAAAAAERAAIQIQMSUf/aAAgBAwEBPwHqU5aq`r`n")
$sbJpgBase64.Append("Axx+y1tMQl4elj//xAAcEQEAAQUBAQAAAAAAAAAAAAABEQACEBIhA1H/2gAIAQIBAT8B3Bhqy7Zc`r`n")
$sbJpgBase64.Append("enyiwmGgDhiOzj//xAAdEAABAwUBAAAAAAAAAAAAAAABAAIREBIhIkFR/9oACAEBAAY/ArZyn+Cg`r`n")
$sbJpgBase64.Append("xtxWuJaoCnqDuin/xAAcEAABBAMBAAAAAAAAAAAAAAABABEhYRAxQVH/2gAIAQEAAT8hkEwPUUR9`r`n")
$sbJpgBase64.Append("DYfE4nxtRpIkBTsayuALIiuY/9oADAMBAAIAAwAAABDWPTsf/8QAGhEAAwADAQAAAAAAAAAAAAAA`r`n")
$sbJpgBase64.Append("AAEREDFBIf/aAAgBAwEBPxC0DVPcWm+Ce4OesrkE6bjH/8QAGBEBAQEBAQAAAAAAAAAAAAAAAREA`r`n")
$sbJpgBase64.Append("QRD/2gAIAQIBAT8QahMiOc8YgSrnTY3ELclHXn//xAAcEAEBAAIDAQEAAAAAAAAAAAABEQAhMUFx`r`n")
$sbJpgBase64.Append("EFH/2gAIAQEAAT8Qn3igmSZSj+c4N4zapMy9IjFV98wncN2iuLFsCEbDGxQkI6RO/n//2Q==`r`n")

$mime = New-Object Chilkat.Mime
$mime.ContentType = "image/jpeg"
$mime.SetBodyFromEncoded("base64",$sbJpgBase64.GetAsString())

$($mime.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.
$mime.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.   
$($mime.GetMime())
$("-")

# We CAN get the binary MIME as bytes..

$mimeBytes = $mime.GetMimeBytes()

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

$jpgBytes = $mime.GetBodyBinary()

# To get the body in base64 format, first make sure
# the Content-Transfer-Encoding is base64, then call GetBodyEncoded.
$mime.Encoding = "base64"
$jpgBase64 = $mime.GetBodyEncoded()
$($jpgBase64)
$("-")

# Let's go back to "binary" MIME..
$mime.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.
$mime.Convert8Bit()
$mimeStr = $mime.GetMime()
$($mimeStr)
$("-")