Why doesn’t the Header Object encode the Filename property using RFC 2047?

The filename property corresponds to the filename parameter of the Content-Disposition MIME field (and sometimes the name parameter of the MIME Content-Type field).

Section 5 of RFC 2047 defines three situations under which encoded-words are allowed in MIME headers and none of these apply to parameters for these fields.

Since using encoded words in these places would result in a non-compliant message the library does not encode the filename.

If you really wish to encode the filename you can manipulate the section's header directly:

set encoder=CreateObject("CSMail.RFC2047")
section.Header("Content-Disposition")="attachment ;filename=" & encoder.encode("ÀBÇ.doc")