Clear your browsing data, cache, etc. from your browser settings. Not sure about other browsers, but Chrome allows you to clear cache and settings at the website level, if desired. However, this may not be necessary. If you can upload attachments without issue, no need to clear anything.
This is what I get on my desktop: The following error occurred There was a problem uploading your file.
Chrome Browser, same computer. Works just fine. Pic of a used crankshaft, "may need cleaning". Not mine by the way.