Should the thumbnail field in a frontend form allow any file type to be uploaded?
I think the thumbnail field is a file upload field type, currently. I'm able to upload a PDF as a thumbnail. Seems like it should be an image upload field type so that it only allows uploading of image file types.
If it is intended to be a file upload field type (allow any type of file to be uploaded), then how can I restrict it to certain file types?
Please don't put any text inside the setCustomValidity function. The content will be output inside quotes via PHP, so it might break the code. Instead of that, please try to add a custom JS to the page (enqueuing a JS file that contains a setCustomValidity() function).