*Contents linked from this page might be in Japanese.
HTML mail to which file is attached, might be in the state where multi part is nested.
The multi part that is not nested is the one which can be handled with the e-mail adapter and the document cannot be acquired from the nested multipart resulting in the output of files other than the attached files.
Check the specification of the mail client who has sent the mail, for the method to make it as the multi part which is not nested.
Refer to help described in the Reference Information for the example of the multi part that is nested.
Help [Adapter] - [Network] - [e-Mail] - [Mail reception (POP3)] or [Mail reception (IMAP4)] - Specification limit