Hi Steve,
There is a thread on this, in which Sanford described a complex javascript method to do this. Do a search for it.
The file would go to Images & Files.
I have some questions you should consider:
- Why do you need to do this?
 - Consider the potential security risk, giving random people the ability to access the system like this.
 - there is a file limit at some point.
 - how will you organize them?
 - how will you connect the file to the lead record?
 - is there another, safer way to do this?
 
Options:
https://community.marketo.com/MarketoDiscussionDetail?id=90650000000PQHIAA4
https://community.marketo.com/MarketoDiscussionDetail?id=90650000000PQe7AAG
https://community.marketo.com/MarketoDiscussionDetail?id=90650000000PWCIAA4