@echadwick-artist There are several issues at play here.
Drag-n-drop support for (sub-)directories is unimplemented. This is certainly possible to add.
Selecting a file and a folder with a system dialog is not possible on the web platform in general. It's either several files from the same directory or one directory with all its content. We'd have to have one more link/button like "Select Directory" to support this workflow.
This applies both to the "Select" link and to the drag-n-dropped files.
Split from https://github.com/KhronosGroup/glTF-Validator/issues/229#issuecomment-2289744430.