Resource interpreted as Stylesheet but transferred with MIME type text/html:

Hi,
I have my css and js files in a sub folder under my root folder :

public_html/sp/mycss.css

under the SP folder thats where i have all my php files.
My home page (Index.php) is in my root folder, my main page is a wordpress themed page, so i added a sub folder in my root folder which has some php files but those files do not load thier css and js files, i get this :

Resource interpreted as Stylesheet but transferred with MIME type text/html:“https://mysite.com/sp/mycss.css”.

Any help will be appreciated thanks!