I wanted to check where you were trying to upload the file, and if the issue persists even after you clear your cache and cookies, or use a different browser?
Running on Windows 10 Firefox Both latest versions, when trying to send any file that is of more than 2-3 mbs, I may be suffocated to upload it around 10 times till this error doesn't get shown. Poor thing that it's 2019 and still Upwork hasn't resolved such a problem stated overtimes over these years. Simply shrugging shoulders and adding a third party button like Dropbox is not enough.
re: "Simply shrugging shoulders and adding a third party button like Dropbox is not enough."
But Upwork freelancers who are providing high-quality service to their clients are able to provide files to them. Regardless of the current state of affairs with regards to Upwork's capabilities.
Upwork truly isn't a file transfer service, and many other services are available. Dropbox. Google Drive. USB drive. TransferWise. Email. FTP.
Dropbox does pretty much nothing for me when it comes to finding clients who want to pay me money, and facilitating payments into my bank account. Nobody is complaining about Dropbox lacking those features.
Often times, a web site code is written in such a way that it works well with some browsers, but not others. And they don't test for all browsers. I have Google Chrome and have Internet Explorer as well. No harm in using both, though preferably not at the same time. I find browsers to be computer memory (ram) hogs. I find Chrome good, but don't open too many tabs in any browser. Right now, Chrome is using 393 Megs of ram with 5 tabs open.
Sometimes shutting down and restarting can be helpful if there is some little glitch or oddity in your system giving you the problem.