How to set sending multiple attachments

Hello everyone.
I searched but did not find.
I have a little problem with the functionality plugin Fast Secure Contact Form.
I’m concerned specifically about adding more attachments on the submission form.
After adding the attachment element you can select only one attachment. Adding attribute “multiple” to select multiple attachments can be, but is not sent in email.
Looking settings and can not figure it out …
Please excuse translator.

Thanks