Using the Contentful Export tool I noticed that you can currently run into a situation where you export assets that have no files linked which will through an error when you try to pass the exported json into the Contentful Import tool as discussed in issue 95.
As a temporary solution I wrote myselve this script that you can pipe data through to filter out these malformed assets.
cat export.json | node filter-empty-assets.js > filtered-export.json
Now you can use the filteres-export.json
for your import and it should work.