Rework downloading web files re naming

[MrCreosote]

Currently the code does

• GET to the source to get the content distribution header, which I assume will also start (and maybe finish) pulling the file from the source
• Inspects the content-distribution header to get the file name
• Otherwise gets the name from the URL
• does another GET to the source using wget to download the file.

Instead

• Use a temporary file name like a UUID
• Download the file
• Figure out the filename as above
• rename the file

Result: 1 less GET