I’m pretty sure that error occurs because itk.js does not see those files as dicom files. Assuming that they are actually dicom files (I am assuming they load up just fine in 3D Slicer or ParaView Glance), can you verify that your web server is serving the dicom files correctly? At the very least, check that files.map((file) => console.log(file.name, file.size)); prints out file sizes that you expect.