Hey Juliet,
Sometimes I have this issue, you can try to clear the cache, or try a different browser.
Mendix doesn't seem to be having issues, but sometimes they take time to report so if the marketplace upload is down on their side try to upload later.
Also check if your file is not corrupted or any special characters in the name since I can't see it.
If you never done it in the past also you can check your permissions if you are allowed to upload a component.
Let me know if any of this helps.