Support file input in form

[hezhenxing]

Please support file input type in form. There is no File type in InputType, and manually adding an input filed of file type does not get the value set in formData.

[seanhess]

Anyone have thoughts on how should we design the interface to this? It's quite different than an InputType: it will upload the file to a temporary location on the server somewhere. We could require the use of WAI middleware, but it might make sense to configure it directly using liveAppWith in the ServerOptions.