Error creatng new script or styles

[jyemail]

Hi Core cms users,

I'm trying to add new scripts or styles in the back-end/ Style/Layout, but core give this message:

File input is registered as a javascript
Fatal error: Detected file as stylesheet, but file did not have the extension .css, it must have.

I could modify an existing file but not create them, some body could tell me what happen, is problem of installation or permissions..

Thanks!..