Vim's filetype for Markdown is "markdown".

README.markdown

@@ -36,16 +36,16 @@ code or execute their script to find them.
 
 At the time of this writing, syntax checking plugins exist for ActionScript,
 Ada, AppleScript, Arduino, AsciiDoc, ASM, BEMHTML, Bro, Bourne shell, C,
-C++, C#, Cabal, Chef, CoffeeScript, Coco, Coq, CSS, Cucumber, CUDA, D, Dart,
-DocBook, Dust, Elixir, Erlang, eRuby, Fortran, Gentoo metadata, GLSL, Go,
-Haml, Haskell, Haxe, Handlebars, HSS, HTML, Java, JavaScript, JSON, JSX, LESS,
-Lex, Limbo, LISP, LLVM intermediate language, Lua, MATLAB, NASM, Objective-C,
-Objective-C++, OCaml, Perl, Perl POD, PHP, gettext Portable Object, OS X and
-iOS property lists, Puppet, Python, Racket, R, reStructuredText, RPM spec,
-Ruby, SASS/SCSS, Scala, Slim, Tcl, TeX, Texinfo, Twig, TypeScript, Vala,
-Verilog, VHDL, VimL, xHtml, XML, XSLT, YACC, YAML, z80, Zope page templates,
-and zsh.  See the [wiki][3] for details about the corresponding supported
-checkers.
+C++, C#, Cabal, Chef, CoffeeScript, Coco, Coq, CSS, Cucumber, CUDA, D,
+Dart, DocBook, Dust, Elixir, Erlang, eRuby, Fortran, Gentoo metadata, GLSL,
+Go, Haml, Haskell, Haxe, Handlebars, HSS, HTML, Java, JavaScript, JSON,
+JSX, LESS, Lex, Limbo, LISP, LLVM intermediate language, Lua, Markdown,
+MATLAB, NASM, Objective-C, Objective-C++, OCaml, Perl, Perl POD, PHP, gettext
+Portable Object, OS X and iOS property lists, Puppet, Python, Racket, R,
+reStructuredText, RPM spec, Ruby, SASS/SCSS, Scala, Slim, Tcl, TeX, Texinfo,
+Twig, TypeScript, Vala, Verilog, VHDL, VimL, xHtml, XML, XSLT, YACC, YAML,
+z80, Zope page templates, and zsh.  See the [wiki][3] for details about the
+corresponding supported checkers.
 
 Below is a screenshot showing the methods that Syntastic uses to display syntax
 errors.  Note that, in practise, you will only have a subset of these methods

plugin/syntastic.vim

@@ -19,7 +19,7 @@ if has('reltime')
     lockvar! g:syntastic_start
 endif
 
-let g:syntastic_version = '3.5.0-45'
+let g:syntastic_version = '3.5.0-48'
 lockvar g:syntastic_version
 
 " Sanity checks {{{1

plugin/syntastic/registry.vim

@@ -51,7 +51,7 @@ let s:defaultCheckers = {
         \ 'lisp':        ['clisp'],
         \ 'llvm':        ['llvm'],
         \ 'lua':         ['luac'],
-        \ 'mkd':         ['mdl'],
+        \ 'markdown':    ['mdl'],
         \ 'matlab':      ['mlint'],
         \ 'nasm':        ['nasm'],
         \ 'nroff':       ['mandoc'],