Module:File link

 -- This module provides a library for formatting file wikilinks. It implements {{file link}}.

 local yesno = require('Module:Yesno')
 local checkType = require('libraryUtil').checkType

 local p = {}

 function p._main(args)
	checkType('_main', 1, args, 'table')

	-- This is basically libraryUtil.checkTypeForNamedArg, but we are rolling our
	-- own function to get the right error level.
	local function checkArg(key, val, level)
		if type(val) ~= 'string' then
			error(string.format(
				"type error in '%s' parameter of '_main' (expected string, got %s)",
				key, type(val)
			), level)
		end
	end

	local ret = {}

	-- Adds a positional parameter to the buffer.
	local function addPositional(key)
		local val = args[key]
		if not val then
			return nil
		end
		checkArg(key, val, 4)
		ret[#ret + 1] = val
	end

	-- Adds a named parameter to the buffer. We assume that the parameter name
	-- is the same as the argument key.
	local function addNamed(key)
		local val = args[key]
		if not val then
			return nil
		end
		checkArg(key, val, 4)
		ret[#ret + 1] = key .. '=' .. val
	end

	-- Filename
	checkArg('file', args.file, 3)
	ret[#ret + 1] = 'File:' .. args.file

	-- Format
	if args.format then
		checkArg('format', args.format)
		if args.formatfile then
			checkArg('formatfile', args.formatfile)
			ret[#ret + 1] = args.format .. '=' .. args.formatfile
		else
			ret[#ret + 1] = args.format
		end
	end

	-- Border
	if yesno(args.border) then
		ret[#ret + 1] = 'border'
	end

	addPositional('location')
	addPositional('alignment')
	addPositional('size')
	addNamed('upright')
	addNamed('link')
	addNamed('alt')
	addNamed('page')
	addNamed('class')
	addNamed('lang')
	addNamed('start')
	addNamed('end')
	addNamed('thumbtime')
	addPositional('caption')

	return string.format('[[%s]]', table.concat(ret, '|'))
 end

 function p.main(frame)
	local origArgs = require('Module:Arguments').getArgs(frame, {
		wrappers = 'Template:File link'
	})
	if not origArgs.file then
		error("'file' parameter missing from [[Template:File link]]", 0)
	end

	-- Copy the arguments that were passed to a new table to avoid looking up
	-- every possible parameter in the frame object.
	local args = {}
	for k, v in pairs(origArgs) do
		-- Make _BLANK a special argument to add a blank parameter. For use in
		-- conditional templates etc. it is useful for blank arguments to be
		-- ignored, but we still need a way to specify them so that we can do
		-- things like [[File:Example.png|link=]].
		if v == '_BLANK' then
			v = ''
		end
		args[k] = v
	end
	return p._main(args)
 end

 return p