So, what's the secret? I see others have gotten past this issue, but not I. Clean build on a Dell R830, virgin hardware. Trying to upload an ISO, and it sits at 0%...
This is usually caused by the SSL Certificate on the ESXi machine not being trusted. You have to add the ESXi host's Certificate to your trusted Certificate authorities on Windows or System key ring and set it to all trusted for the issue to go away (and restart your browser).
And ESXi 6.5 is already out, you can try that instead of ESXi 6.0U2