Allow If-Match header.

When updating an item If-Match header with etag must be send.
This will allow it for CORS.
Implement upload endpoint.

There is /upload/ endpoint for file uploads.
On upload we get url to fetch the file.

SDUX-38 #resolve
Keep file extension on upload for mimetype detection.

When uploading file, use original extension so we can
guess mimetype and use it on download.

SDUX-38