Unfortunately this is not currently supported but it would be a very useful feature. Currently they must be declared in DLL. From the documentation we have:
header: when set to true the first line of files will be used to name columns and will not be included in data. All types will be assumed string. Default value is false.
which is what you are seeing.
Note that it is possible to infer schema at query time, e.g.
select sum(mystringfield) from mytable