This may not solve your problem, but I ran into an issue with CSS files in Tridion where the multimedia type had the wrong name between environments. In my case, it was stopping content porter, but it's possible that the multimedia type has something off in it:
If you're uploading the CSS file as a binary, follow these steps.
Go to Administration > Multimedia Types
Cascading style sheet
If it says
Cascading style sheets, you may want to change it:
change the Name field appropriately.
My guess, though, is that John is probably right and either a schema doesn't have CSS as an allowed type, or that the CSS is corrupted.