According to the RFC 7231 section 3.1.5.5:
A sender that generates a message containing a payload body SHOULD generate a Content-Type header field in that message unless the intended media type of the enclosed representation is unknown to the sender. If a Content-Type header field is not present, the recipient MAY either assume a media type of “application/octet-stream” ([RFC2046], Section 4.5.1) or examine the data to determine its type.
It means that the Content-Type
HTTP header should be set only for PUT
and POST
requests.