Shaun: you should ask the freelancer to provide the file to you through some other method, such as FTP, email, Dropbox, etc.
Upwork interferes with files in ways unknown to your freelancer and outside of your freelancer's control. It does this for security reasons and to avoid spreading malware, among other reasons. But its protocols can indeed interfere with image files. I have personally experienced problems with image file transmission.
I looked into this further, and I was unable to replicate your issue. When you go to your contract page, you can click on Milestones & Payments > Completed Milestones > Click on the file (the one with the paperclip) > Work Submissions > Attachment.
Please also try clearing your cookies, or using a different browser to see if the issue persists.