When uploading, upload status indicator is unpredictable/unreliable

Description

Steps to reproduce

1. From an Alloy site > CMS > Edit.
2. Start the upload of 20 files (~140kb each) to media.
3. The upload progress bar goes to 100% (green) right away, which indicates it is done.
4. At the same time, the label says "uploading ..."

Expected: The green progress bar goes to complete after last file is uploaded.