Hello jennibelize,
Thank you for reaching out to Apple Support Communities.
Let's test in a private window to see if the same thing happens. These steps can guide you:
"Test with a private window
A website can store cookies, caches and other data on your Mac, and issues with that data can affect your use of the website. To prevent the website from using that data, view it in a private window: From the menu bar in Safari, choose File > New Private Window, or press Shift-Command-N.
If that works, use the following steps to remove the website's data, including its caches and cookies. The website can then create new data as needed. If it's a website that you sign in to, make sure you know your sign-in information before continuing.
- Choose Safari > Settings (or Preferences), then click Privacy.
- Click Manage Website Data.
- Select the affected website from the list that's displayed.
- Click Remove.
- Click Done.
- Open the website again in a non-private browser window."
Additional details can be found here: If Safari won't open a page or work as expected on your Mac
If the issue persists, can you provdie a few more details? Can you explain in detail what steps you are taking to upload using safari? This will help us further isolate the issue. Also, does this happen on multiple websites.
Cheers!