Doesn't download file types

[hadess]

For example, the stand-alone waf file here I would expect to be downloaded as the filename mentioned for building.

                {   
                    "type": "file",
                    "url": "https://waf.io/waf-1.8.12",
                    "sha256": "01bf2beab2106d1558800c8709bc2c8e496d3da4a2ca343fe091f22fca60c98b",
                    "dest-filename": "waf"
                },  

[hadess]

Note that I just realised that we'd be missing a way to set the executable bit on the downloaded file.

[alexlarsson]

This issue was moved to flatpak/flatpak#8