Fix downloads route error handling

[nmurrell07]

Corrected usage of req.params.file in downloads example.

[krzysdz]

The file parameter is a wildcard parameter, which means that in req.params it is an array (in Express 5). For more information see #5885, pillarjs/router#117 or the documentation.

P.S. The commit shows as authored by "virtual machine".

[nmurrell07]

Submit from a VM. I will review feedback and resubmit.