Use concrete types in MimeExt (#109)

Code generated to work on this Dict shows up as a minor source
of invalidation when loading packages that define new
`AbstractString` types.

src/Mime_ext.jl

@@ -2,7 +2,7 @@ module Mime_ext
 
 export MimeExt
 
-MimeExt = Dict{AbstractString,AbstractString}(
+const MimeExt = Dict{String,String}(
     "ez" => "application/andrew-inset",
     "anx" => "application/annodex",
     "atom" => "application/atom+xml",