You must have mistaken Extension Manager. Extension Manager CS6 only registers itself for .ZXP, .MXP and .MXI file types. It will never register any other file types. You can follow the steps below to check the file type association:
1. Run /Applications/Utilities/Terminal.app.
2. Copy/paste the following command into Terminal then press "Enter" key:
/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.fra mework/Versions/A/Support/lsregister -dump > ~/Documents/dump.txt
3. Open dump.txt in your Documents folder. Search "Extension Manager CS6", you would see only .mxp, .zxp and .mxi are shown in bindings.
4. You can check which applications have bindings upon .CSS, .XSL, etc through dump.txt as well.
I'd like to agree that I was mistaken, but Extension Manager did take over file types for several scripting files. I changed CSS files back a few days ago so those are again showing as text files, but I also saw it as the default app for opening .conf files and .xsl files. See screenshot attached - Extension Manager icon on .xsl file. I have no idea why it was the default. No one else uses my computer, and I have never personally launched the Extension Manager program. All files were showing as type "Adobe Extension Manager Script" or similar, before I changed them all back to use a text editor as default. The icons are slowly changing back as they become registered correctly again.
Same thing happened to me. It was a css file that was highjacked. Thanks Adobe.