chickadee » spiffy » mime-type-map

An alist of extensions (strings) to mime-types (symbols), to use for the content-type header when serving up a static file. Defaults to

 '(("html" . text/html)
   ("xhtml" . application/xhtml+xml)
   ("js"  . application/javascript)
   ("css" . text/css)
   ("png" . image/png)
   ("xml" . application/xml)
   ("pdf" . application/pdf)
   ("jpeg" . image/jpeg)
   ("jpg" . image/jpeg)
   ("gif" . image/gif)
   ("ico" . image/vnd.microsoft.icon)
   ("svg" . image/svg+xml)
   ("txt" . text/plain))

See also file-extension->mime-type for a procedure which can look up file extensions for you.