Phone downloads mysterious file I wasn't doing anything with my phone, when I pulled it out of my pocket a notification saying "-tWW3IHZgSg - Download complete." was visible. This file was in the downloads folder, I copied it to my PC and running the unix "file" command on it returned "Macromedia Flash data (compressed), version 17". I certainly didn't download it and I have no idea what it could be. Here is a DropBox link of the file: https://www.dropbox.com/s/yowi4qlnlxuf6wz/-tWW3IHZgSg?dl=0
Possibly a flash animation file. Without access to my computer, I can't find out for certain.

As for why it showed up in your downloads? Most likely an error in a program that caused a download notification when downloading cache data or a temporary file, judging by the filename. Probably an advert or small animation from a webpage or app. 
It's worth noting that most free apps have ads as well as most websites, and most advert animations are flash animations. 

TL;DR: Probably a glitch causing an advert or cached page animation to appear in your downloads.