Can't add fields to Multipart body headers

**Romuald** ([Bug 49949](https://bz.apache.org/bugzilla//show_bug.cgi?id=49949&redirect=false)):
When using the HTTPClient tool, we can attach a file or many and name it but in some occasions we would be interested in adding header aside from the usual Content-Disposition and Content-Type, Content-Transfer-Encoding.

Some applications like SAP with their Archivelink HTTP interface are adding fields there (X-compId as an example).

Therefore for such applications it is not possible to simulate a file upload with a POST request as the backend webserver is not able to manage it (doesn't receive the missing header field).

In the attached screenshot I have highlighted in yellow the piece that I would like to configure the same way we can do with the HeaderManager for example.

Hope I didn't use bugzilla by mistake as I didn't find another location to post this enhancement request. If it is the case I am more than happy to post it at the right place.

Thanks.

Created attachment [Capture.JPG](https://apache.github.io/jmeter-bugzilla-attachments/49/49949/26041/Capture.JPG): Screenshot of a HTTP Request dump with MS NETMON 3.3

OS: Windows Vista

No source files or tests are named. Start by locating the HTTP Request multipart handling and compare it with HeaderManager configuration; done means allowing extra per-part headers such as X-compId and confirming that they are included in the multipart POST.

Written by the indexing model from the issue text.