2.1.43 [HTML] Section 17.13.4, Form content types
The specification states:
As with all multipart MIME types, each part has an optional "Content-Type" header that defaults to "text/plain". User agents should supply the "Content-Type" header, accompanied by a "charset" parameter.
All Document Modes (All Versions)
The Content-Type header is omitted except when an INPUT element has type =
file. For INPUT elements of type =
file, the Content-Type is provided, but charset is omitted and a default of UTF-8 encoding is used.