fix missing supported extension check in case when asset is a string
fix missing supported extension check in case when asset is a string

I've came across this inconsistent logic while investigating my own issue: https://github.com/storybooks/storybook/issues/2363
storybooks/react-storybook,rhalff/storybook,storybooks/react-storybook,storybooks/storybook,storybooks/storybook,kadirahq/react-storybook,storybooks/storybook,rhalff/storybook,storybooks/storybook,rhalff/storybook,storybooks/storybook,rhalff/storybook,rhalff/storybook,rhalff/storybook,storybooks/react-storybook,storybooks/storybook,storybooks/react-storybook,kadirahq/react-storybook,storybooks/storybook