Revert "remove default extension handling"
Revert "remove default extension handling"

This reverts commit aea13797485d0b73f6e0aa95119dbafab8dcc220.

synaptek/builder,alexisvincent/builder,jrauschenbusch/builder,systemjs/builder,systemjs/builder,robwormald/builder,richardTowers/builder,richardTowers/builder,Psvensso/builder,alexandr2110pro/builder,alexisvincent/builder,alexisvincent/builder,buddhike/builder,Munter/builder,douglasduteil/builder,jrauschenbusch/builder,whitneyit/builder,buddhike/builder,jrauschenbusch/builder,lookfirst/builder,Psvensso/builder,douglasduteil/builder,whitneyit/builder,synaptek/builder,lookfirst/builder,Munter/builder,systemjs/builder,robwormald/builder,alexandr2110pro/builder