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SOLUTION!

    InputStream in = new FileInputStream(file);
    byte[] buf;
    buf = new byte[in.available()];
    while (in.read(buf) != -1) {

    }
    RequestBody requestBody = RequestBody
            .create(MediaType.parse("application/octet-stream"), buf);

and send the created requestBody as the body of the POST. In my case, I am using RxJava, so the ApiService call looks like:

@POST(ApiUrls.Endpoint.UPLOAD_IMAGE_ATTACHMENT)
Observable<Void> uploadImageAttachment(
        @Header(ApiUrls.Param.X_REQUEST_DIGEST) String digestValue,
        @Path(ApiUrls.Param.RESULT_ID) int resultId,
        @Path(ApiUrls.Param.FILE_NAME) String fileName,
        @Body RequestBody photo);

and send the created requestBody as the body of the POST.

and send the created requestBody as the body of the POST. In my case, I am using RxJava, so the ApiService call looks like:

@POST(ApiUrls.Endpoint.UPLOAD_IMAGE_ATTACHMENT)
Observable<Void> uploadImageAttachment(
        @Header(ApiUrls.Param.X_REQUEST_DIGEST) String digestValue,
        @Path(ApiUrls.Param.RESULT_ID) int resultId,
        @Path(ApiUrls.Param.FILE_NAME) String fileName,
        @Body RequestBody photo);

SOLUTION!

    InputStream in = new FileInputStream(file);
    byte[] buf;
    buf = new byte[in.available()];
    while (in.read(buf) != -1) {

    }
    RequestBody requestBody = RequestBody
            .create(MediaType.parse("application/octet-stream"), buf);

and send the created requestBody as the body of the POST.