Chart data cannot be added to display sets via UI

[miriam-groeneveld]

Describe the bug
When creating or updating a display set, selecting a socket of kind Chart renders no usable widget to add the data.

To Reproduce
Steps to reproduce the behavior:

1. Go to the Cases of any reader study
2. Click on Edit display set or Add display set
3. Select a socket with kind Chart, for example ROC Curve or FROC curve.
4. No widget is rendered.

Expected behavior
A widget to upload JSON data.

Screenshots

Additional context
Sockets with kind Chart that expect a JSON file, for example the ECG socket, do show the correct (image select or image upload) widget. The difference being the setting for store_in_database.
Uploading the JSON data via gcapi does work, this only concerns upload via UI.