(Delphi ActiveX) 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 for Delphi Downloads Chilkat ActiveX DLL for Delphi Chilkat non-ActiveX DLL for Delphi * The examples here use the ActiveX DLL.

uses
    Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
    Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, Chilkat_TLB;

...

procedure TForm1.Button1Click(Sender: TObject);
var
success: Integer;
sbJpgBase64: TChilkatStringBuilder;
mime: TChilkatMime;
mimeBytes: Array of Byte;
jpgBytes: Array of Byte;
jpgBase64: WideString;
mimeStr: WideString;

begin
// 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 := TChilkatStringBuilder.Create(Self);
sbJpgBase64.Append('/9j/4AAQSkZJRgABAQEASABIAAD//gAmRmlsZSB3cml0dGVuIGJ5IEFkb2JlIFBob3Rvc2hvcD8g' + #13#10);
sbJpgBase64.Append('NC4w/9sAQwAQCwwODAoQDg0OEhEQExgoGhgWFhgxIyUdKDozPTw5Mzg3QEhcTkBEV0U3OFBtUVdf' + #13#10);
sbJpgBase64.Append('YmdoZz5NcXlwZHhcZWdj/9sAQwEREhIYFRgvGhovY0I4QmNjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj' + #13#10);
sbJpgBase64.Append('Y2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj/8IAEQgAFAAUAwERAAIRAQMRAf/EABcAAAMBAAAA' + #13#10);
sbJpgBase64.Append('AAAAAAAAAAAAAAIDBAX/xAAYAQADAQEAAAAAAAAAAAAAAAABAgMEAP/aAAwDAQACEAMQAAAB2kZY' + #13#10);
sbJpgBase64.Append('NNEijWKddfTmLgALWH//xAAbEAACAgMBAAAAAAAAAAAAAAABAgMRAAQSE//aAAgBAQABBQL0XqN+' + #13#10);
sbJpgBase64.Append('pM2aqJGMiqFFCyg7z//EABwRAAICAgMAAAAAAAAAAAAAAAERAAIQIQMSUf/aAAgBAwEBPwHqU5aq' + #13#10);
sbJpgBase64.Append('Axx+y1tMQl4elj//xAAcEQEAAQUBAQAAAAAAAAAAAAABEQACEBIhA1H/2gAIAQIBAT8B3Bhqy7Zc' + #13#10);
sbJpgBase64.Append('enyiwmGgDhiOzj//xAAdEAABAwUBAAAAAAAAAAAAAAABAAIREBIhIkFR/9oACAEBAAY/ArZyn+Cg' + #13#10);
sbJpgBase64.Append('xtxWuJaoCnqDuin/xAAcEAABBAMBAAAAAAAAAAAAAAABABEhYRAxQVH/2gAIAQEAAT8hkEwPUUR9' + #13#10);
sbJpgBase64.Append('DYfE4nxtRpIkBTsayuALIiuY/9oADAMBAAIAAwAAABDWPTsf/8QAGhEAAwADAQAAAAAAAAAAAAAA' + #13#10);
sbJpgBase64.Append('AAEREDFBIf/aAAgBAwEBPxC0DVPcWm+Ce4OesrkE6bjH/8QAGBEBAQEBAQAAAAAAAAAAAAAAAREA' + #13#10);
sbJpgBase64.Append('QRD/2gAIAQIBAT8QahMiOc8YgSrnTY3ELclHXn//xAAcEAEBAAIDAQEAAAAAAAAAAAABEQAhMUFx' + #13#10);
sbJpgBase64.Append('EFH/2gAIAQEAAT8Qn3igmSZSj+c4N4zapMy9IjFV98wncN2iuLFsCEbDGxQkI6RO/n//2Q==' + #13#10);

mime := TChilkatMime.Create(Self);
mime.ContentType := 'image/jpeg';
mime.SetBodyFromEncoded('base64',sbJpgBase64.GetAsString());

Memo1.Lines.Add(mime.GetMime());
Memo1.Lines.Add('-');

// 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.   
Memo1.Lines.Add(mime.GetMime());
Memo1.Lines.Add('-');

// 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();
Memo1.Lines.Add(jpgBase64);
Memo1.Lines.Add('-');

// 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();
Memo1.Lines.Add(mimeStr);
Memo1.Lines.Add('-');
end;