Revert "Fix overuse of FormData in XmlHttpRequest"
Revert "Fix overuse of FormData in XmlHttpRequest"

It breaks sending files with CHrome using XmlHttpRequest

This reverts commit 8e0dad8738d210fca2c1df6a65ea3ef3dd52b8ab.

alainfrisch/js_of_ocaml,alainfrisch/js_of_ocaml,OCamlPro-Henry/js_of_ocaml,OCamlPro-Henry/js_of_ocaml,OCamlPro-Henry/js_of_ocaml,alainfrisch/js_of_ocaml,alainfrisch/js_of_ocaml,alainfrisch/js_of_ocaml,OCamlPro-Henry/js_of_ocaml,OCamlPro-Henry/js_of_ocaml