Closes #1888; Add mimetye image/vnd.microsoft.icon to mimtype list

packages/rocketchat-assets/server/assets.coffee

 sizeOf = Npm.require 'image-size'
 mime = Npm.require 'mime-types'
 
+mime.extensions['image/vnd.microsoft.icon'] = ['ico']
+
 @RocketChatAssetsInstance = new RocketChatFile.GridFS
 	name: 'assets'