I have the same issue. With other file hosting platforms we can use HEAD requests and check the Content-Type header to confirm URLs are valid and are going to serve a PDF or PNG images. This avoids having to use a GET request and the time involved to download the image. We process 10K's orders per day and we rely on HEAD requests to quickly confirm if order content and image URLs submitted are valid. This avoids extra time involved with GET to download images and validate.
HEAD requests for Box image URLs return a 404 instead of responding with a 200 and a Content-Type header.
Can the Box platform be configured to support HEAD requests for public URLs?