prevent "local variable 'output_format' referenced before assignment" error.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@42345 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5/mixin/downloadable.py

@@ -80,6 +80,7 @@ class DownloadableMixin:
       format = None
     self._checkConversionFormatPermission(format, **kw)
     mime, data = self.convert(format, **kw)
+    output_format = None
     if not format:
       # Guess the format from original mimetype
       mimetypes_registry = getToolByName(self.getPortalObject(),