use png as default image format, because unlike jpg it supports transparency

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@22937 20353a03-c40f-0410-a6d1-a30d3c3de9de

bt5/erp5_base/SkinTemplateItem/portal_skins/erp5_base/File_viewFieldLibrary/my_thumbnail.xml

@@ -262,7 +262,7 @@
                 </item>
                 <item>
                     <key> <string>image_format</string> </key>
-                    <value> <string>jpg</string> </value>
+                    <value> <string>png</string> </value>
                 </item>
                 <item>
                     <key> <string>image_resolution</string> </key>

bt5/erp5_base/bt/revision

-406
\ No newline at end of file
+407
\ No newline at end of file