As you can see (From the error) that the file received is:

Providers\HtmlEditorProviders\Fck\FCKeditor\editor\fckeditor.html

and it was sent by your server to your browser as is (That is the way it works). The server also adds a content-type header when generating the response.

Then, that file must be received by your browser and must by formated as an html file.

So, the two possible problems your have:

1) You browser is not sending the appropriate content-type

2) Your browser is not rendering the file as expected.

As you can see. The file was sent and the only task accomplished by provider/editor is to give your server the correct path of that file. The server must send that file and your browser must draw the content.

Sorry. I don't know how to help you with this. Hope you can find a solution.