content-type altered

Using Unirest, I am trying to upload a PDF file to a third party program. This third party program requires the Content-Type to be set like so: `application/pdf;name=test.pdf` however when I inspect the network traffic I see that Unirest is removing `;name=test.pdf` from the Content-Type so that it is only sent with `application/pdf`. Is there any way to prevent this?

Here is the full code:

```
const unirest = require('unirest');
const fs = require('fs');
const path = require('path');

var Request = unirest.post('http://192.168.1.200:95/Insert');

Request
.attach('remote file', fs.createReadStream(path.join(__dirname, 'test.pdf'))) // Attachment
.headers({
'Content-Type': 'application/pdf;name=test.pdf',
})
.end(function (response) {
console.log(response.body);
});
```

Start by reproducing the request shown in the issue and inspect how Request.attach() and headers() are serialized into the outgoing multipart request. Trace the request-building path to find where the Content-Type parameter is removed. Done means the outgoing request preserves application/pdf;name=test.pdf and the existing upload behavior remains intact.

Written by the indexing model from the issue text.