Now I can give this a 5 stars since I have finally found an easy to use fix for users that have the problem about images getting saved without an file-extension! (especially images from Twitter) All that's needed is this batch file:

---
@echo off
color 0a

REM Change drive to D:/ because my files are there
REM If yours are on C, you can remove this and change all Ds to Cs
cd /d d:

REM Move only files WITHOUT an extension to TempDirectory
MOVE D:\Downloads\*. D:\TempDirectory\

REM Change directory to TempDirectory
cd D:\TempDirectory

REM Rename all files from "file" to "file.jpg"
ren *.* *.jpg

REM Move files back to origin folder
MOVE D:\TempDirectory\* D:\Downloads

EXIT
---

I know it's not good practice to assume all images are JPEGs but it's enough to view them, which is all I wanted.