Description:
------------
if you give CURLFile's an empty string as the third argument, it will in fact swap out the empty string with the first argument!

that makes porting this curl command rather difficult:

curl -F "uploadManifest={json};type=application/json" http://127.0.0.1:9999/

i guess it's debatable weather emptystring should remove the filename header entirely (like CURLOPT_HTTPHEADER does, for example to remove libcurl-generated "Expect: 100-continue" headers, set CURLOPT_HTTPHEADER=>array("Except: "), and the Expect header won't be generated at all), or if it should literally send filename=""
(but my personal opinion is that it should work the same as CURLOPT_HTTPHEADER, eg remove the filename header entirely), but whatever the right action is, it's certainly not the current action of replacing it with the on-disk file-location, and that's what it's currently doing. (maybe that would be ok if argument 3 is null, i don't know, but it's not appropriate if the 3rd argument is emptystring, the programmer specifically asked for an empty filename, and now doesn't get one.)

Test script:
---------------
<?php
$stupid_workaround_fileh = tmpfile();
$stupid_workaround_filef = stream_get_meta_data($stupid_workaround_fileh)['uri'];
fwrite($stupid_workaround_fileh,"{json}");
$ch=curl_init();
curl_setopt_array($ch, array(
    CURLOPT_URL => "http://127.0.0.1:9999/",
    CURLOPT_POST => 1,
    CURLOPT_POSTFIELDS => array(
        'uploadManifest' => new CURLFile($stupid_workaround_filef, 'application/json', '')
    )
));
curl_exec($ch);

Expected result:
----------------
POST / HTTP/1.1
Host: 127.0.0.1:9999
Accept: */*
Content-Length: 186
Content-Type: multipart/form-data; boundary=------------------------2e0011350c342f21

--------------------------2e0011350c342f21
Content-Disposition: form-data; name="uploadManifest"
Content-Type: application/json

{json}
--------------------------2e0011350c342f21--


Actual result:
--------------
POST / HTTP/1.1
Host: 127.0.0.1:9999
Accept: */*
Content-Length: 214
Content-Type: multipart/form-data; boundary=------------------------2e0011350c342f21

--------------------------2e0011350c342f21
Content-Disposition: form-data; name="uploadManifest"; filename="/tmp/phpSh32lX"
Content-Type: application/json

{json}
--------------------------2e0011350c342f21--