Can an attachment be sent from a web2lead form into Salesforce lead?

You can have attachments added to Leads (you can enable that on the setup of the Leads page layout object).

As for adding attachments from the web form itself - I am not aware of a standard setting, but a short google search, brought 2 options

http://boards.developerforce.com/t5/General-Development/web-to-lead-form-with-attachment/td-p/336137

• The second one is from FormAssebmly