[galaxy-dev] Specifying data types for workflow inputs

I am developing a custom workflow in galaxy. I have 4 input files that need to be fed into various steps of the workflow. I would like to be able to specify a data type for the inputs such that the pre-populated drop downs are filtered to datasets of the appropriate type as well as being able to do some simple validation before the workflow begins. Is there someway to do this? Thanks much, Ann