Fixes error when uploading not allowed files

To reproduce:

(we are using it on rails admin but I think it is the same on a standard instance of ck editor)

1. Click on "Link"
2. Switch to "Upload" tab
3. Add a `.txt` file
4. Click on "Send it to the server"
5. Click "OK"

You will get an error in the console:
![screen shot 2014-12-01 at 17 30 32](https://cloud.githubusercontent.com/assets/556268/5249030/3cf94f04-7980-11e4-8b28-daad8132d370.png)


The reason is the unescaped quote here:
![screen shot 2014-12-01 at 17 31 06](https://cloud.githubusercontent.com/assets/556268/5249039/4a75f97a-7980-11e4-9d00-7baf38760dbe.png)



This PR solves this issue.

I wanted to add a failing test but it would require too much time on my side, sorry