File-list editor widget quietly returns an empty file value if uploaded file extension is invalid

[bferguso]

When uploading a file, if the extension of the uploaded file is invalid (ie - it doesn't match the list of valid extensions in the config), the widget quietly emits an empty file value object rather than either displaying an error, or disallowing the file drop.

Additionaly - if the extension types are specified, there is no indication in the widget what they are.

[johnatawnclementawn]

Drag & drop will prevent adding a file with an invalid file type, but will not surface an error after #274