Improve ClipboardChromium image file name handling

Use image decoder file extension when naming image files rather than
the response content type. Preserve any provided extension e.g. jpeg
if it has the same mime type as the image decoder file extension.

Test: http/tests/images/drag-drop-image-filename.html

BUG=259669

Review URL: https://chromiumcodereview.appspot.com/19520004

git-svn-id: svn://svn.chromium.org/blink/trunk@154602 bbb929c8-8fbe-4397-9dbb-9b2b20218538