I had the same issue. I don't know why, but I changed the attachment from mp4 to zip format and was able to upload it.
CAT-liter
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That was my mistake. The file name should have been ".png" instead of ".PNG".
But I had no problem using "*.PNG" until last month...
In my experience, I've never gotten a response from a reviewer no matter what I ask. The only way I know of to communicate with a reviewer is to resubmit the app with a note in App Review Information. (I think all developers feel frustrated communicating with reviewers)