When enabling "Apply Date, Author and Dublin Core properties.", the files attached to "file:content" don't have a mime-type after import, resulting in failed thumbnail generation
If the import mode is not activated, then the mime-type is set correctly by the platform.
In the code it looks like file:content is treated differently than other blob properties and cannot accept an object including the mime-type like described at https://doc.nuxeo.com/nxdoc/nuxeo-csv/#files-attachment. Thus there is no workaround.