Uploading and using data to a geoprocessing service

Hy,
I'm trying to build a simple geoprocessing service which should buffer an uploaded shapefile.


This is the scenario:
1) uploading shapefile to geoprocessing service
2) buffering the shapefile with a given parameter
3) returning the buffered shapefile

The buffering is not the problem, but the handling of the upload and reusing it in the model or python part.
As far as I read, I should have to use a Datafile parameter for upload, but I'm always getting this error while running it:
ExecuteError: Failed to execute. Parameters are not valid. ERROR 000735: input: Value is required Failed to execute


Can anybody help me to use uploads on geoprocessing services?


Thanks,
Tom