You can write a small User Control which contain asp.net's FileUpload control - on postback you will save the file somewhere of your choosing and the send a string value (where you saved the file) up as your result to the forms renderer. The data that gets saved contain the path and you have the file stored - I guess that would work?
See http://docs.composite.net/Console/CreatingFormControls for a sample on how you can register your own tags in the C1 Forms markup - this particular example is about adding new tags to the forms in the C1 Console, but the 'forms system' core is common for Forms.Renderer and C1 Forms markup - they are just two different channels (you will see these two channels when you edit Composite.config in the before mentioned guide.
See http://docs.composite.net/Console/CreatingFormControls for a sample on how you can register your own tags in the C1 Forms markup - this particular example is about adding new tags to the forms in the C1 Console, but the 'forms system' core is common for Forms.Renderer and C1 Forms markup - they are just two different channels (you will see these two channels when you edit Composite.config in the before mentioned guide.