I have created some number of my thumbnailers in directory /usr/share/thumbnailers/ such as this:
[Desktop Entry]
Version=1.0
Encoding=UTF-8
Type=X-Thumbnailer
Name=Wine Thumbnailer
MimeType=application/x-msdownload;application/x-ms-dos-executable;
X-Thumbnailer-Exec=/usr/local/bin/wine-thumbnailer %i %o %s
It's fine works in Xfce 4.6, but in version 4.8 legacy format of thumbnailers is broken and for writting the simple thumbnailer(execute external command) i need known C and research header files of tumbler. This is a unpleasant situation for some people. I think, creating of *.desktop (or more simpler format?) plugin to tumbler is not a hard task.