It appears that when a resource URL is using dropbox for any of the following: images, videos, pdfs, Safari is not able to render image, pdf (or play video).

Steps to replicate:

1. Upload a video, pdf, image in your Dropbox account and make URL publicly available.
2. Create a course and turn edit mode to on.
3. Add a Resource URL.
4. Set the External URL to any of the files you have from step1
5. Use Firefox browser.
6. RESULT: All of them are viewable and rendered correctly
7. Use Safari browser
8. RESULT: None of them are viewable.