don't try to convert non images

bt5/erp5_dms_conversion_catalog/SkinTemplateItem/portal_skins/erp5_dms_conversion_catalog/Image_preConvert.xml

@@ -57,6 +57,9 @@ if quality is None:\n
   # it\'s required so fall back to system preferences as\n
   # directly accessed over URL will do the same\n
   quality = context.getDefaultImageQuality(format)\n
+if not context.getContentType().startswith(\'image/\'):\n
+  context.log(\'%s is not an image, skipping preconversion\' % context.getRelativeUrl())\n
+  return \n
 \n
 # UI uses \'large\' display\n
 display_list.append(\'large\')\n

bt5/erp5_dms_conversion_catalog/bt/revision

-24
\ No newline at end of file
+25
\ No newline at end of file