Change csv extractor filename trait type

It helps in the GUI because it displays the correct widget for file names

force_bdss/core_plugins/dummy/csv_extractor/csv_extractor_model.py

-from traits.api import Int, String, on_trait_change
+from traits.api import Int, String, on_trait_change, File
 
 from force_bdss.api import BaseDataSourceModel
 
 
 class CSVExtractorModel(BaseDataSourceModel):
-    filename = String()
+    filename = File()
     row = Int()
     column = Int()
     cuba_type = String()