I got 2 SP2010 (win 2008R2) farms and 2 web-applications running on them:
Farm1 (prd) -> app-prd Farm2 (tst) -> app-tst
(app-prd and app-tst are FQDN http links)
on those web-applications we've created one site collection (in / path) and deployed custom solution with some application, both sites are using the same version of this custom solution,
one site (app-prd) renders ok, second site (app-tst) is rendering with no css, ugly "icon map" and some unwanted texts visible to end-users; administrative pages are also without css,
after some checking i discovered that on app-tst I see errors in IE/Firefox console:
CSS was ignored due to mime type mismatch
for 2 css files
the css files are served as application/octet-stream and on other collecions as text/css
i've created app-tst/site/1 site collection for testing purpose but it works just fine,
on IIS Mime-Types i see proper entry - .css -> text/css
tried iisreset and pool recycling with no luck,
i am running out of ideas :)