We'll create fresh WordPress site with Signature Field For Contact Form 7 – CF7Sign installed. You have 20 minutes to test the plugin after that site we'll be deleted.
Contact Form 7 signature (CF7 signature) plugin helps you to add signature fields.
Add sign tag from wp-admin > contacts > slect form > add tag [sign]
Example: [sign your-sign]
Note: Never wrap with <label> tag.
Good:-
<label> Signature </label>
[sign* sign-568]
bad:-
<label> Signature
[sign* sign-568]</label>
Then add input field name [your-sign] in contact form 7 > mail > attachments section.
This plugin compatible with contact form 7 database plugins
CF7 Database