Fixes unencoded spaces in PreviewHttpPath for the FTP uploader.

This is a quick fix for #364.

I noticed that you can't use paths that contain URL encoded characters like `/imgs/New%20Folder` or non-ascii characters like `/imgs/アニメ` from user input. It looks like things have been that way since before I started tinkering with this, so I'll create an Issue for it and work on it when I get a chance.