I am developing a Win8 (WinRT, C#, XAML) client application (CSOM) that needs to download/upload files from/to SharePoint 2013.
How do I do the Download/Upload?
Just a suggestion SharePoint 2013 online & on-prem file encoding is UTF-8 BOM. Make sure your file is UTF-8 BOM, otherwise your uploaded html and scripts may not rendered correctly in browser.