I just tested this, and all videos I tested are being shown with the correct proportions now. And Thunar was able to show thumbnails to all videos unless WMV videos.
After install gstreamer1.0-plugins-ugly the WMV videos had thumbnails and worked properly, with their proportions right.
NOTE: the Parole window that asks to install the codecs did not work, I had to install the package gstreamer1.0-plugins-ugly manually with apt.
I just tested this, and all videos I tested are being shown with the correct proportions now. And Thunar was able to show thumbnails to all videos unless WMV videos.
After install gstreamer1. 0-plugins- ugly the WMV videos had thumbnails and worked properly, with their proportions right.
NOTE: the Parole window that asks to install the codecs did not work, I had to install the package gstreamer1. 0-plugins- ugly manually with apt.
To me looks like this problem is fixed now.