Gallery macro displays the wrong thumbnail when two attachments share the same filename but different file types.

http://extranet.atlassian.com/display/QA/Confluence+2.8+Macro+Examples
Two of the attachments of this page are;
EFY_Bamboo.jpeg
EFY_Bamboo.png

The first image has a black background, the latter has a white background. The thumbnail displayed in the gallery macro for the second image is the correct thumbnail for the first image.