added support for SVG mime-type recognition. Closes #493

`convert` will pick this up automatically. :)

lib/paperclip/upfile.rb

@@ -11,6 +11,7 @@ module Paperclip
       when %r"jp(e|g|eg)"            then "image/jpeg"
       when %r"jp(e|g|eg)"            then "image/jpeg"
       when %r"tiff?"                 then "image/tiff"
       when %r"tiff?"                 then "image/tiff"
       when %r"png", "gif", "bmp"     then "image/#{type}"
       when %r"png", "gif", "bmp"     then "image/#{type}"
+      when %r"svg"                   then "image/svg+xml"
       when "txt"                     then "text/plain"
       when "txt"                     then "text/plain"
       when %r"html?"                 then "text/html"
       when %r"html?"                 then "text/html"
       when "js"                      then "application/js"
       when "js"                      then "application/js"

test/upfile_test.rb

@@ -6,6 +6,7 @@ class UpfileTest < Test::Unit::TestCase
     %w(png)          => 'image/png',
     %w(png)          => 'image/png',
     %w(gif)          => 'image/gif',
     %w(gif)          => 'image/gif',
     %w(bmp)          => 'image/bmp',
     %w(bmp)          => 'image/bmp',
+    %w(svg)          => 'image/svg+xml',
     %w(txt)          => 'text/plain',
     %w(txt)          => 'text/plain',
     %w(htm html)     => 'text/html',
     %w(htm html)     => 'text/html',
     %w(csv)          => 'text/csv',
     %w(csv)          => 'text/csv',