This help page explains some common reasons for upload problems and how to resolve them:
That said, we have noticed reports of random upload issues affecting a tiny minority of users lately and been trying to track down the reason. Unfortunately, there's no clear cause or any common factor we've been able to identify so far, but it seems to be related to upstream network issues (e.g., ISP connectivity or router hardware) rather than hardware/software issues on our users' own computers.
If possible, you might try rebooting your router/modem (if there's no power switch, just pull the power plug, wait ~10 sec., then plug it back in), or retry the upload from another location with a different Internet connection. If that's not practical or still doesn't work, you can email our support team for an alternate method of providing file(s) for us to upload for you: