Apparently that's not the only file you need to edit. In fact, you don't really even need to edit the Extensions.txt because that's just an aesthetics file used for displaying cute icons and stuff. What you really need to edit is the MMDocumentTypes file (it doesn't have an extension on it), located in the DocumentTypes folder inside the Configuration folder where the other text file is located.
Find this line:
<documenttype id="JavaScript" internaltype="Text" winfileextension="js,json" macfileextension="js,json" file="Default.js" writebyteordermark="false" mimetype="text/javascript">
Replace it with this (just removing the json
part from it):
<documenttype id="JavaScript" internaltype="Text" winfileextension="js" macfileextension="js" file="Default.js" writebyteordermark="false" mimetype="text/javascript">
Reload Dreamweaver and it will not think that your JSON files are JavaScript anymore and will display it simply as a text file (no syntax highlighting or checking, just plain black text).