You cannot just rename files to change the format.
Here's how to convert a whole bunch of files:
1. Rename them back to what they were.
2. Open Photoshop.
3. Select the whole bunch of files and drag them to the top border of Photoshop.
4. Go through the icons on the left of the Camera Raw dialog and make whatever settings you want for each file (or all the files) so that they look good to you.
5. Click the link at the bottom-center of the Camera Raw dialog and ensure the color space and size are what you want. I suggest using the sRGB color space for JPEG files.
6. Click the [Select All] button, then click the [Save Images...] button at the bottom left.
7. Choose the Folder, Format, etc. that you want to use to save the files.
8. Click the [Save] button.
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On a separate note, if you don't want JPEGs (and TIFF files) to automatically open into Photoshop using Camera Raw, do this:
1. Open Photoshop.
2. Choose Edit - Preferences - Camera Raw...
3. Change the values in the JPEG and TIFF Handling section to "Disable support".
Thanks so much - this is exactly what I needed! You saved me a bunch of time!!!!!