Allowing document upload to form

Unfortunately, no.
https://success.salesforce.com/answers?id=90630000000gsPIAAY

Well, not without a very complex workaround:
http://blog.soquelllc.com/?p=1

One other option would be to use the Gravity Forms SalesForce Add=on Plugin:
https://wordpress.org/plugins/gravity-forms-salesforce/

However, the attachment would stay on your site (in Gravity Forms) — not be uploaded to SalesForce as the Web 2 Lead API does not allow uploads.

You’d need a Enterprise account and Force.com “site” to allow direct file uploads — apparently.