This seems to show a bug with the DataFileReader

gistfile1.txt

import java.io.File;
import java.io.FileOutputStream;
import java.io.OutputStream;

import org.apache.avro.Schema;
import org.apache.avro.file.DataFileReader;
import org.apache.avro.file.DataFileWriter;
import org.apache.avro.file.FileReader;
import org.apache.avro.generic.GenericDatumReader;
import org.apache.avro.generic.GenericDatumWriter;
import org.apache.avro.io.DecoderFactory;

public class Hrm {
    public static void main(String[] args) {
        try {
            String schema = "{\"fields\": [{\"type\": [\"null\", \"bytes\"], \"default\": null, \"name\": \"field1\"}, {\"type\": [\"null\", \"string\"], \"default\": null, \"name\": \"field2\"}], \"type\": \"record\", \"name\": \"test\"}";
        	  String record = "{\"field1\": {\"bytes\": \"i am a banana\"}, \"field2\": {\"string\": \"i am a banana\"}}"; 

            Schema avroSchema = new Schema.Parser().parse(schema);

            GenericDatumReader<Object> reader = new GenericDatumReader<Object>(avroSchema);
            
            DataFileWriter<Object> writer = new DataFileWriter<Object>(new GenericDatumWriter<Object>(avroSchema));
            File output = new File("test.avro");
            output.delete();
            OutputStream os = new FileOutputStream(output);
            writer.create(avroSchema, os);

            Object datum = reader.read(null, DecoderFactory.get().jsonDecoder(avroSchema, record));
            writer.append(datum);
            writer.close();
            os.close();
            
            FileReader<Object> dfr = DataFileReader.openReader(new File("test.avro"), new GenericDatumReader<Object>());
            System.out.println(dfr.next()); // {"field1": {"bytes": "i am a banana"}, "field2": "i am a banana"} !
        } catch (Exception e) {
            throw new RuntimeException(e);
        }        
    }
}