2 big ways I would recommend fixing this.
Make sure your device is plugged in and the screen stays turned on during long uploads, and don't browse other websites or visit other apps. Mobile devices will aggressively shut down pages/apps you haven't looked at in a while to save memory, but if you're actually in the middle of something, like uploading a file, this can break things.
Or upload the files from a PC or laptop that won't try to shut down your web pages to save memory.
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