mitsuhiko/werkzeug — comment on commit 29ca958cd8b3a17f0cb613030dac16efe0a49ea3
File: werkzeug/formparser.py line 326
By mitsuhiko:

The answer is easier. If unspecified the browser follows the encoding the page is in. Unless you change that this will always be UTF-8 in Werkzeug since this is out default outgoing encoding.