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AttachmentEncoding

property AttachmentEncoding: string read write;

Example​

procedure UseMIMEAttachmentEncoding;
var
  Msg: TIdMessage;
begin
  Msg := Email;
  if Msg = nil then Exit;

  Msg.Encoding := meMIME;
  Msg.AttachmentEncoding := 'MIME';
end;

Usage​

AttachmentEncoding selects the message-parts encoder registry entry used for attachment layout, with Encoding assignments able to overwrite the selection.

Additional Technical Info​

AttachmentEncoding is a forwarding property over the AttachmentEncoding property of MessageParts; the message does not keep a second attachment-encoding value. Reading it returns the selected Indy message-encoder registry name. A new parts collection initially selects MIME.

Assignment performs a process-wide encoder-registry lookup before storing the name. Unknown or unregistered values raise EIdException, and lookup-before-store leaves the previous encoder selection intact. This property is therefore not an arbitrary Content-Transfer-Encoding header.

Interaction with Encoding​

Assigning Encoding also assigns this property:

  • meMIME selects MIME;
  • meDefault or mePlainText selects UUE.

Set Encoding first and apply a deliberate attachment override afterwards. During plain-text layout, an attachment with an empty ContentTransfer uses this preference only when it is UUE or XXE; otherwise that path falls back to UUE. MIME output normally defaults an attachment's empty transfer encoding to base64 while it writes the part.

Changing this property selects encoder metadata for later generation. It does not traverse or re-encode existing parts, refresh cached part counts, or update each part's ContentTransfer. GenerateHeader and send/save processing can subsequently normalize or fill part encodings.

The registry and message are shared mutable state without script-level synchronization. Configure the message before another operation starts serializing it. The example is source-reviewed only and was not sent or executed.

External references

Created 2026-07-15