mime: please add 'text/x-markdown' for markdown file extensions (md, markdown) to the _defaultExtensionMap

[DartBot]

Originally opened as dart-lang/sdk#12857

This issue was originally filed by ross.dart....@gmail.com

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0.6.21_r26639

As markdown is quite common these days, and this change does not conflict with anything currently in the _defaultExtensionMap, I think it is reasonable to add it?

'md':'text/x-markdown',
'markdown':'text/x-markdown',

The mime type 'text/x-markdown' seems to be what is most commonly used, for example:

node-mime: https://github.com/broofa/node-mime/pull/48/files
CodeMirror: http://codemirror.net/mode/markdown/

thanks!

[DartBot]

This comment was originally written by ross.dart...@gmail.com

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hmm actually while you're in there (if you go in) can you also please add 'yaml'? I'm not sure what the appropriate mime type for it is, there seems to be less agreement for it...

[DartBot]

<img src="https://avatars.githubusercontent.com/u/5449880?v=3" align="left" width="48" height="48"hspace="10"> Comment by iposva-google

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Added Area-Pkg, Library-MIME, Triaged labels.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/3276024?v=3" align="left" width="48" height="48"hspace="10"> Comment by anders-sandholm

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Removed Library-MIME label.
Added Pkg-MIME label.