When using the multipart/form-data, libsoup uses quotes around the boundary parameter value.

This causes responses with status-code 404 from some servers I try to upload files to. 

According to specifications, this is not invalid, but RFC 2616 section 19.2 has the following note:
<cite>
2) Although RFC 2046 [40] permits the boundary string to be
   quoted, some existing implementations handle a quoted boundary
   string incorrectly.
</cite>

As the boundary string used by libsoup is a hexadecimal string, there are no reason to quote the boundary string.

This is easily fixed in soup-multipart.c by changing a single line in function soup_multipart_to_message ()
From:
	content_type = g_strdup_printf ("%s; boundary=\"%s\"",
To:
	content_type = g_strdup_printf ("%s; boundary=%s",