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.