How to transform file Content-Type from Multipart/form-data to application/pdf ?

Hi,

I have a big issue with file content-type during uploading the files into Messagebird server.
After successful uploaded into Messagebird server, but the file content-type always in Multipart/form data.
But the requirement from Messagebird the Content-Type must in application/pdf . Therefore I tried with postman to test and it was perfect result. But not sure where is my mistake from the code. Below is my custom function :

downloadfile = invokeurl

[

url: "https://www.zohoapis.com/crm/v3/Deals/"+ dealID + "/Attachments/XXXXX"

type: GET

connection:"downloadfile"

];

// Uploading file attachment to Messagebird Server

head = Map();

head.put("Authorization","AccessKey XXXXXX");

head.put("Content-Type","application/pdf");

head.put("Content-Disposition","attachment;filename=Ticket_Voucher.pdf");

messagebird = invokeurl

[

url :"https://messaging.messagebird.com/v1/files"

type :POST

headers:head

files: downloadfile

];

info messagebird;

fileID = messagebird.get("id");

info "https://messaging.messagebird.com/v1/files/" + fileID;

Below is the requirement from Messagebird

https://developers.messagebird.com/api/file-storage/#upload-file

After uploaded successful, i use websniffer.com to check the files and the result always in Multipart/form-data but not application/pdf.

I need help from all the expert to get rid of this problem.
Thank you