2.1.67 [W3C-HTML51] Section 4.10.22.7 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.

EdgeHTML Mode

Forms are always encoded as UTF-8.