Reblogging this for reference…
An issue I ran into during an upgrade to SharePoint 2019 with custom branding was that the style sheets weren't loaded correctly by the browser. The console logs showed these resources being downloaded as application/octet-stream mime type rather than text/css.
Stefan Goßner has a blog post about this explaining that the mime type cannot be determined properly for files hosted in document libraries. Currently a workaround exists that enables the IIS 6 Metabase Compatibility Windows Feature:
A fix is in the works and should resolve the issue permanently without the need for the workaround.