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

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

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: ""+ dealID + "/Attachments/XXXXX"
type: GET

// Uploading file attachment to Messagebird Server
head = Map();
head.put("Authorization","AccessKey XXXXXX");
messagebird = invokeurl
type :POST
files: downloadfile
info messagebird;
fileID = messagebird.get("id");

Below is the requirement from Messagebird

After uploaded successful, i use 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

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