return bytes, not BytesIO object for preview

domdiv/draw.py

@@ -28,6 +28,7 @@ class DividerDrawer(object):
     def __init__(self):
         self.odd = True
         self.canvas = None
+        self.options = None
 
     @staticmethod
     def get_image_filepath(fname):

domdiv/main.py

@@ -512,7 +512,7 @@ def generate_sample(options):
     with Image(blob=buf.getvalue(), resolution=options.preview_resolution) as sample:
         sample.format = 'png'
         sample.save(sample_out)
-        return sample_out
+        return sample_out.getvalue()
 
 
 def parse_papersize(spec):