e.g.
ABC.jpg is ok
ABC.JPG can be selected, is shown in the file manager preview, is stored correctly, can be used in a layout etc. but won't show in the Custom Field Media JCE preview
J!4.1.5, JCE Pro 2.9.27
Additional question:
What is the upload button for? I is only showing up on existing Custom Fields, it's not there on "new" ones (created with + in repeatable subforms)?
I can select an image for uploading (looks like a standard HTML upload) but then nothing happens.
I think it should not be there at all, the user should only be able to upload via file/image manager with resize settings etc.