Only guess mimetype if mime is not empty.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@42179 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5/mixin/downloadable.py

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