Use InputStream instead of File for DW server file type

[kelnos]

File requires that Jersey write out the data to a temp file on disk, while InputStream allows Jersey to stream the data to us.

I'm not sure how well it'll work client-side. Users will have to create a FileInputStream to pass instead of a File (which isn't a big deal), but unclear how to make AsyncHttpClient work with it. AHC has InputStreamBodyGenerator for turning an IS into a request body, but doesn't have something analogous for multipart file parts.

[blast-hardcheese]

Something similar could be used in scala land as well. File was a stopgap to get anything working. The helper method that generates Files from known parameters per handler route could be altered to produce InputStreams