2.1.58 [W3C-HTML52] Section 22.214.171.124. Multipart form data
V0207: Forms are always encoded as UTF-8
The specification states:
... Multipart form data The multipart/form-data encoding algorithm is as follows: ... 2. If the algorithm was invoked with an explicit character encoding, let the selected character encoding be that encoding. ... ... Otherwise, let the selected character encoding be UTF-8.
Forms are always encoded as UTF-8.