If the question is how to read all files from hdfs directory,
in general, each file is potentially a different DataSet (not DataStream).
It needs to be decided how to combine/join them in Flink pipeline.
If the files are small enough, you could list them as string paths and use env.fromCollection to start the pipeline.
Next just manually load file into memory for each path in map operation and transform file contents into records for the next stage.