Renamed variable for first part of splitext to root

[radarhere]

Suggestions for python-pillow#8338

• https://docs.python.org/3/library/os.path.html#os.path.splitext lists the first part of splitext as "root", so this renames "filename_body" to that
• Rather than writing to a tmp_path, just use tempfile.NamedTemporaryFile()