Whats Content-Type value within a HTTP-Request when uploading content?

The content type is per specification multipart/form-data.

This is a special content type which can be visualized as multiple sub-requests in one big request. Each of those sub-requests (one form-data element) has their own set of headers. The content type of the actual data is in there.

Here's an example how it look like with 1 normal field and 1 file field (in HTML terms, when using <input name="textfield"><input type="file" name="filefield">):

Content-Type: multipart/form-data;boundary=SOME_BOUNDARY

--SOME_BOUNDARY
content-disposition: form-data;name="textfield"
content-type: text/plain;charset=UTF-8

value of textfield here
--SOME_BOUNDARY
content-disposition: form-data;name="filefield";filename="some.ext"
content-type: application/octet-stream

binary file content here

--SOME_BOUNDARY--

As to parsing and extracting this data, practically every programming language has builtin/3rd party APIs for this. As you didn't tell anything about which one you're using, it's impossible to give a targeted answer. In case of for example Java, that would be either the 3rd party library Apache Commons FileUpload or when you're using Servlet 3.0, the API-provided request.getPart() method.