Allow for PUTing to /username/table.csv

[calpaterson]

Brief overview

AS A user PUTing some table

I WANT to be able to just PUT with an extension

SO THAT I can upload in cases where it's hard to add an HTTP header

Additional details

Particularly relevant for parquet, for which there is no agreed mimetype (though that in itself requires some work: https://github.com/calpaterson/csvbase/issues/99)

Not hard with our current codebase, just needs a small rejig