I've been hesitant to file this bug for months, but recently I was in Central America and ran into this bug constantly. Sorry I don't have more technical details.

Steps to reproduce:

1. Be on a slow internet connection. I know that's vague, but let's say >500kbps for upload speed.
2. Try to upload a file that is 2MB or larger using Special:UploadWizard or Special:Upload.

Expected results: File uploads successfully after about 2-3 minutes.

Actual results: File uploads for about 2 minutes and then fails with the message "Could not proceed due to network connectivity issues. Make sure you have a working internet connection and try again."

I have no idea why 2MB seems to be the magical threshold, but I was usually able to upload smaller files, but rarely anything 2MB or larger. I had no problems doing the same thing on other sites like Dropbox, although it might take forever.

Some logs of trying to upload 2 files that are 1.7 MB and 2MB:

When it finally fails, it throws the following console error:
https://commons.wikimedia.org/w/api.php net::ERR_SPDY_PING_FAILED

This is in Chrome 72.0.3626.121 on MacOS. Download speed ~1.2 Mbps and upload speed ~0.4 Mbps.