Only show the body in the asset drawer for text assets
Only show the body in the asset drawer for text assets

Non-text assets, such as embedded charts will look crazy in the asset
drawer. Only render the body for text assets and use the default icon
for everything else.

Addresses #770

denverfoundation/storybase,denverfoundation/storybase,denverfoundation/storybase,denverfoundation/storybase