The Upwork messenger tool automatically makes modifications to jpeg files. The way it transmits files can be dififcult to predict.
If transmitting files to clients is an important part of your work, I would urge you to familiarize yourself with other means of transmitting files, such as Dropbox, FTP, email, Google Cloud, etc.
It is okay to use Upwork to transmit files, but it isn't a guaranteed method and it won't work with all file types or file sizes. You should be aware of the limitations of using Upwork for file transmission, and you should have other ways of doing.
Preston: thank you! I've had a few problems recently and only this morning went to Dropbox ... it would be good if Upwork could communicate this potential problem to freelancers so we don't have to learn the hard way (and inconvenience clients). That would be much more professional in the long run.