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mptAttachment

mptAttachment = 1

Example​

procedure InspectPart(Part: TIdMessagePart; var IsAttachment: Boolean);
begin
  IsAttachment := Part.PartType = mptAttachment;
end;

Usage​

mptAttachment classifies a Velox message part as a base or attachment runtime class for counting and Velox file-download handling.

Additional Technical Info​

mptAttachment is the attachment result of the virtual TIdMessagePart.PartType method. TIdAttachment returns it explicitly; the base class also returns it, so custom descendants are classified as attachments unless they override the method.

The result is based on runtime class behavior, not Content-Type, filename or Content-Disposition. The example is source-reviewed and is not executed by the documentation workflow.

Counting and structure

TIdMessageParts.CountParts rescans the collection and increments its cached AttachmentCount for this result. A Content-ID independently increments RelatedPartCount, so one item can be both an attachment-class part and related content. Counts remain stale after collection mutation until another scan.

Velox POP/IMAP behavior

The automatic email-receive loops select only parts returning mptAttachment, then cast them to TIdAttachmentFile. That cast matches Indy's default receive behavior: without a custom attachment factory, the decoder creates file-backed attachments, and Velox does not install a custom factory.

After classification Velox still requires a matching configured filename pattern and a nonempty filename before saving. Unnamed inline/signature parts are skipped. Therefore mptAttachment means eligible runtime class, not that Velox will create an output file.

Custom code must not assume every custom subclass returning this value is TIdAttachmentFile; test/know the concrete class before a downcast. A part labelled text/plain can still return Attachment.

Side effects, errors and performance

Reading PartType is constant-time and has no I/O or mutation. Later casts, filename processing, attachment streams and file saves can fail. The borrowed part becomes invalid if its owning collection/message is destroyed; do not mutate the collection concurrently while iterating.

Related entries

External references

Created 2026-07-15