fix #4344: http-server: ignore unused multipart headers

fix #4344: http-server: ignore unused multipart headers

In commit 0fbcbc2 ("fix #3990: multipart upload: rework to fix
uploading small files") a breaking change was added which now
requires the file's multipart part to have a `Content-Type` even
though the content type is never used. It is just included to consume
those bytes so phase 2 (dumping the file contents into the file) can
continue.

Avoid this overly strict and unused requirement.

Signed-off-by: John Hollowell <jhollowe@johnhollowell.com>
 [ T: resolve merge conflict, add telling commit message ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>