`require.extensions` is unfortunately a badly designed API because you can set it at any point in time. This makes a lot of optimizations impossible like parsing files in parallel or before hand. It is also [deprecated by node](http://nodejs.org/api/globals.html#globals_require_extensions) but without any alternative.

Instead, could you add support for a `moduleFileExtensions` configuration the same way `testFileExtensions` is working. The setup isn't compatible with node but the execution is. I think it's the best trade-off. What do you think?