I can't get the image upload to work in the frontend. Any attempt to upload results in a red triangle over the thumbnail with an exclamation mark. Clicking the triangle creates a popup which says 'Upload error, validation.extensions' with the option to remove file.
Is this a known problem or a configuration problem on my end? Every other form element works perfectly.
Speech about "fileupload"? Judging by the text of the error, the problem is with the extension of the uploaded file (validation of the file uploaded to the server does not pass). Check all possible limitations and settings of the fileupload widget (fileTypes, mimeTypes, maxFilesize). Also check the allowed extensions on the server side (Apache, Nginx ...)
Not sure what was causing the problem but I have reinstalled a fresh site and now the image upload is working. However, it seems you can only have one upload widget in a form. A simple test with a form with just two file uploads, let's say photo1 and photo2. Upload an image to photo1 and save the form. Then check and you will find that the image has been assigned to photo2 and not photo1.