Fix upload remaining time

The upload remaining time is always 'a few second' whatever a big or a
small file uploading.
This commit fixes it. The `new Date().getMilliseconds()` only return a
three digits number. When time arrived the next second, the millisecond
start from ZERO again. So `new Date().getTime()` is the righe choice.
And remaining time variables shoule be initialized when the file starts
uploading, otherwise the remaining time of a new upload will always be
'Infinity years' until you refresh the page.

Signed-off-by: Yaojin Qian <i@ume.ink>