POST a binary data file to 3rd party application

Hi, I am new to the REST API in Salesforce. I have to POST a binary csv file to the 3rd party application and the given below is curl example: curl --request POST \ --header 'Authorization: Bearer <token> \ --header 'Content-type: text/csv' \ --header 'Accept: text/plain' \ --data-binary @/myhome/salesdata-2018-03-26.csv \ https://admin.testsite.com/hapi/merchant/<id>/sales-data/api And here is my apex code: String fileName = 'salesdata-' +date.today().Day() +'-'+ date.today().Month() +'-'+ date.today().Year()+'.csv'; String targetURL = 'https://entufl5t6i91i.x.pipedream.net'; // assemble the body payload String header = 'Content-Disposition: form-data; name="file"; filename="' + fileName + '"; Content-Type: application/octet-stream; '; String headerEncoded = EncodingUtil.base64Encode(Blob.valueOf(header+'\n\n')); String bodyEncoded = EncodingUtil.base64Encode(csvBlob); Blob bodyPayload = EncodingUtil.base64Decode(headerEncoded+bodyEncoded); // send out the request HttpRequest req = new HttpRequest(); req.setBodyAsBlob(bodyPayload); req.setHeader('Authorization', 'bearer hjghj'); req.setHeader('Content-Type', 'text/csv'); req.setMethod('POST'); req.setEndpoint(targetURL); req.setTimeout(60000); Http http = new Http(); HttpResponse response = http.send(req); system.debug('response: ' + response); catch(Exception e) system.debug('e:' + e);

And the request bin URL is https://requestbin.com/r/entufl5t6i91i/1d4zZyJdIarNLef7Gq5SlveeRih
And it does not look right.

When I POST it to actual 3rd part application the response is 400 and the message is 'bad request'. Kindly point out what I am doing wrong here.

Many thanks!