I removed symlink created by myself (rm /usr/lib/libpython2.7.so.1.0), downgraded python-caja to 1.8.0-1 with apt-get install python-caja/testing and got my extension working again.
For me it seems that package flle layout of 1.8.1-1 is wrong, but package file layout of 1.8.0-1 is right. I have faced similar issue before with gir1.2-caja package in Ubuntu (see https://bugs.launchpad.net/bugs/1380313). So I think that python stuff should be placed directly in /usr/lib (not /usr/lib/i386-linux-gnu) directory. If you can, please test the python-caja package by yourself (with my caja-thg python extension or other Caja extension).
