test/template/asset_tag_helper_test fails on win32

compute_public_path is appending a period (.) even when ext is nil.

source += ".#{ext}" if ext && File.extname(source).blank? || File.exist?(File.join(ASSETS_DIR, dir, "#{source}.#{ext}"))

Should be

source += ".#{ext}" if ext && (File.extname(source).blank? || File.exist?(File.join(ASSETS_DIR, dir, "#{source}.#{ext}")))

On windows, appending periods (.) to the end of a filename still refers to the same file. So if File.exist?("rails.png") returns true, so will File.exist?("rails.png.") and File.exist?("rails.png.....").

So File.exist?(File.join(ASSETS_DIR, dir, "#{source}.#{ext}"))) will always be true for a file that exists. Nothing should be appended if ext isn't specified.