When serving output files, intelligently determine the appropriate content-type.

cherry pick 6fa3fda3ed28239cf3ac9720629f9707663aa197 from datajoy.

app/coffee/ContentTypeMapper.coffee

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+Path = require 'path'
+
+# here we coerce html, css and js to text/plain,
+# otherwise choose correct mime type based on file extension,
+# falling back to octet-stream
+module.exports = ContentTypeMapper =
+	map: (path) ->
+		switch Path.extname(path)
+			when '.txt', '.html', '.js', '.css'
+				return 'text/plain'
+			when '.csv'
+				return 'text/csv'
+			when '.pdf'
+				return 'application/pdf'
+			when '.png'
+				return 'image/png'
+			when '.jpg', '.jpeg'
+				return 'image/jpeg'
+			when '.tiff'
+				return 'image/tiff'
+			when '.gif'
+				return 'image/gif'
+			when '.svg'
+				return 'image/svg+xml'
+			else
+				return 'application/octet-stream'