gspot is pretty good at detecting an avi when it sees one. I suspect your file may be a fake. You can do one last quick check using a a hex editor like
http://www.chmaas.handshake.de/delphi/freeware/xvi32/xvi32.htm#download. If it’s an avi, at the start of the right column you should see RIFF AVI LIST. If you see nothing but zeroes it’s definitely a fake! It could be another file type... for example PK indicates a zip, MZ an .exe, JFIF a jpeg etc...