GpgOL from gpg4win-3.0.0-beta276/ does not detect some OpenPGP NO-MIME emails.
Thus it does not offer to decrypt them or the attachment.
It is assumed that this was due to some MIME content-type like text/html and multipart/related
where not searched for strings that indicate Open-PGP NO-MIME contents.
The original assumption was that GpgOL cannot handle base64 transfer encoded OpenPGP NO-MIME emails (some call it pgp/inline) messages. If such a mail arrives, the base64 encoded content of the message will be decrypted correctly by Outlook, but the gpg encrypted content won't be recognized by GpgOL, thus there will be no decryption of the original message.
To create such a mail, create a symmetric gpg encrypted text, base64 encode it. Write an EMail with some random content in KMail to yourself and encrypt and sign it. Sent it to yourself and save the sent mail as "mbox"-File. Delete the content of the mail and replace it with some symmetric gpg and base64 encoded text and modify the headers to
Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: base64
as properties. Send this mail to your Outlook account, where GpgOL is installed. You will see the plain OpenPGP armored text, so the base64 decryption worked, but there will be no attempt to decrypt the message itself.