Expand checkurl to support recommended filename, and multi-file-urls

This commit was sponsored by an anonymous bitcoiner.

Types/UrlContents.hs

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+{- git-annex URL contents
+ -
+ - Copyright 2014 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Types.UrlContents where
+
+import Utility.Url
+
+data UrlContents
+	-- An URL contains a file, whose size may be known.
+	-- A default filename will be provided, and can be overridded
+	-- or built on.
+	= UrlContents (Maybe Integer) (FilePath -> FilePath)
+	-- Sometimes an URL points to multiple files, each accessible
+	-- by their own URL.
+	| UrlNested [(URLString, UrlContents)]