handle error if Folder included in upload

If a client uploads a selection of objects, and the selection contains a Folder, a TypeError is generated for each object, and the upload can't complete.

Decompress the attached zip file, select the three standalone PNG files and 'fol1', and drag them to the import window – all four objects will be marked with a TypeError. Cancel, hard reload, and try again without 'fol1' – the three remaining objects are processed as expected.

This is not a request to handle folder uploads – basically the inclusion of a folder in a drag and drop selection should not poison the rest of the upload.