specify backgroud color explicitly because recent ImageMagick generates transparent PNG from PDF.

bt5/erp5_dms/SkinTemplateItem/portal_skins/erp5_dms/PDF_renderHTMLPreviewAsImage.xml

@@ -41,7 +41,7 @@
                    python:here.PDF_getPageNumberFromSelectionIndex(request.get(\'selection_index\', 0));\n
                  display python: request.get(\'display\', context.portal_preferences.getPreference(\'preferred_image_size\', \'large\'));\n
                  format python: request.get(\'format\', context.portal_preferences.getPreference(\'preferred_image_format\', \'png\'))">\n
-  <img src="" alt=""\n
+  <img src="" alt="" style="background-color:white;"\n
       tal:attributes="src\n
         string:${here/absolute_url}/index_html?display=${display}&format=${format}&frame=${frame}"/>\n
 </div>

bt5/erp5_dms/SkinTemplateItem/portal_skins/erp5_dms/PDF_renderHTMLThumbnailPreview.xml

@@ -43,12 +43,11 @@
               format python: request.get(\'format\', \n
                                context.portal_preferences.getPreference(\'preferred_image_format\', \'png\'))">\n
   <a tal:attributes="href string:PDF_viewHTMLPreviewAsImage?selection_index=${frame}">\n
-    <img title="Thumbnail"\n
+    <img title="Thumbnail" style="background-color:white;"\n
          i18n:attributes="title" i18n:domain="ui"\n
          tal:attributes="src string:index_html?display=${display}&format=${format}&frame=${frame}"/>\n
   </a>\n
-</tal:block>\n
-
+</tal:block>
 
 ]]></unicode> </value>
         </item>

bt5/erp5_dms/bt/revision

-1256
\ No newline at end of file
+1257
\ No newline at end of file