prevents you from

AttributeError: 'NoneType' object has no attribute 'get' (the none object is "attrs")

other possibility:
if attrs is not None:
    self.attrs = attrs.copy()
else:
    self.attrs = {}

seen @ django.forms.widgets.Widget

filebrowser/fields.py

@@ -25,7 +25,7 @@ class FileBrowseWidget(Input):
     class Media:
         js = (os.path.join(URL_FILEBROWSER_MEDIA, 'js/AddFileBrowser.js'), )
 
-    def __init__(self, attrs=None):
+    def __init__(self, attrs={}):
         super(FileBrowseWidget, self).__init__(attrs)
         self.site = attrs.get('filebrowser_site', None)
         self.directory = attrs.get('directory', '')