If you can, avoid using TextEdit to muck around in system files. BBEdit or TextWrangler is okay for that. (It is very easy to have an accidental keypress leave an invisible (non-printing) character that wreck the whole file.
There is a direct and fairly easy way to use the os system built-in editors to do this:
1) Have your board-id from your config.plist file handy.
2) Open a Terminal window (Terminal.app is in your /Applications/Utilities folder
3a) At the prompt, type cd
/System/Library/PrivateFrameworks/AppleVPA.framework/Versions/A/Resources/ and press the return key
3a1) Note: If you are familiar with Terminal, then drag and drop from a finder window will work for the cd command.
3b) Type ls -la and press the return key, you should get something that looks like this:
drwxr-xr-x 5 root wheel 160 Oct 18 15:42 .
drwxr-xr-x 5 root wheel 160 Oct 18 15:42 ..
drwxr-xr-x 3 root wheel 96 Aug 20 22:00 English.lproj
-rw-r--r-- 1 root wheel 5337 Oct 18 22:26 Info.plist
-rw-r--r-- 1 root wheel 513 Oct 11 23:52 version.plist
3c) Type sudo nano info.plist and press the return key. You will be asked to enter your password. Do so and press the return key.
4) You will be in a text editor that actually looks and acts like an older version of TextEdit. Okay, a very old version.
5) Press the control key and the w key at the same time to bring up the 'where is' search function. Paste in or type in the board id that you have been holding off to the side all of this time from step 1). Press the return key.
You will be taken to the line that has your board-id in it.
6) Use the cursor arrow keys on your keyboard to move to the entry in your board id section for the jpeg = true part. Change this to false.
7) Press the control key and the o key at the same time to bring up the 'write out' file option (save) and just press the return key to save the file using the default info.plist suggested file name.
8) Exit the editor by pressing the control and x keys together at the same time.
9) Reboot the machine and you should have jpegs again in QuickLook and Preview.
Alternatively, you can delete the entire section for your board-id and get the desired results, but ... why?
Note: After each update, you will have to reapply this technique, the updater rewrites the info.plist file.