Use the filetype: search operator.
However,
When you use the filetype: operator in a Google search (for example, filetype:xml galway), Google searches for files with the .xml extension, not for files of file type XML.
Not all URLs or pages have the .html file extension. Some pages (like http://www.google.com) have no extension at all, and some have an extension that doesn't match their file type. Limiting your search to a specific file type will return pages with that file extension, and may return fewer relevant results.
Source.
filetype:cssandfiletype:jsin the query? – Alex Aug 30 '12 at 17:11filetype:csssearches URLs that have the extension ".css", and likewise for ".js". It doesn't search files that have the associated mime types. So the pages returned in afiletype:csssearch are all forward facing pages in the visible content of the website, they aren't the external CSS files that are linked to from the HTML document. – w3d Aug 30 '12 at 17:58