email attachment problem

I would like to know if “transport/email/filename” is the only parameter to check whether a file is attached to an email or not. The filename is null for processing body and also for the scenario of no file attached. I am trying to branch on filename to send a notification if no attachment is send. Currently I am getting 2 of these emails. Do I need to change some settings to avoid this. My current listener settings are as below:

Global Service (optional) getAttachmentService
Default Service (optional) unspecified
Send reply email with service output No
Send reply email on error No
Delete valid messages (IMAP only) No
Delete invalid messages (IMAP only) No
Multithreaded processing (IMAP only) No
Number of threads if multithreading turned on 0
Invoke service for each part of multipart message Yes
Include email headers when passing message to content handler No
Email body contains URL encoded input parameters No


How to reduce service threads