Currently, the list of extensions supported by unpacker is hard-coded. The only way to use an unpacker on files with an extension that is not in the list is to open the files manually, which is not possible when WinMerge is invoked by a VCS tool like TortoiseSVN.
So it would be nice to be able to add extensions to the list. For instance, there are many file formats based on XML, with various extensions, but DisplayXmlFiles.dll only recognizes a few of them. Making the list configurable would fix the problem.