There isn't a way to filter by file type when importing into the bar-descriptor.xml, but you can modify the bar-descriptor.xml to filter out files or directories when building.
If you add the folder c:\MyApp\Assets to your bar-descriptor.xml file and open its source you'll see something like this:
<asset path="C:/Users/msohm/Downloads/Assets">Assets</asset>
You can modify this to exclude certain files or directories. Wild cards are also supported. The example below is modified to filter out all files with a txt extension.
<asset path="C:/MyApp/Assets">Assets
<exclude name="*.txt"></exclude>
</asset>
You can read more about this here.
