fix python 3 compatibility issue, trim trailing space

flask_admin/contrib/fileadmin.py

@@ -847,7 +847,7 @@ class FileAdmin(BaseView, ActionsMixin):
             helpers.flash_errors(form, message='Failed to edit file. %(error)s')
 
             try:
-                with open(full_path, 'r') as f:
+                with open(full_path, 'rb') as f:
                     content = f.read()
             except IOError:
                 flash(gettext("Error reading %(name)s.", name=path), 'error')