:: Predefined attachments ::..

You need to upload attachment to the server and then point to the file from the form using such tag: <input type=hidden name="_attachment_1" value="/path/to/file">. To attach this file to the email template you need to put such line after header information:

Attachment: [_attachment_1]

Example:
Form will have:
<input type=hidden name="_attachment_1" value="../www/attachments/test.txt">

Email Template will have such lines:
To: info@MitriDAT.com
From: <[re_eMail]>
Subject: Attachments Form - Demo
Attachment: [_attachment_1]
….

Will attach test.txt file to the email.

Another way is just to point to the file name located at the attachments directory [file must be located at the directory defined at the personal config file].

Example:
Email Template will have such lines:
To: info@MitriDAT.com
From: <[re_eMail]>
Subject: Attachments Form - Demo
Attachment: test.txt
….

Will attach test.txt file to the email.

:: User attachments ::..

Form tag:
<input type=file name="field_name">

eMail template:
Attachment: [field_name_uploaded]

Example:
Form has such tag:
<input type=file name="User-File">

Email Template will have such lines:
To: info@MitriDAT.com
From: <[re_eMail]>
Subject: Attachments Form - Demo
Attachment: [User-File_uploaded]
….

Script will attach user file "User-File" to the email template.

:: Combinations and Multiple attachments ::..

To have more attachments you need to separate them in the email template using comma separation.

Example:
To: info@MitriDAT.com
From: <[re_eMail]>
Subject: Attachments Form - Demo
Attachment: [_attachment_1], [User-File_uploaded], test.txt