I am not able to call a #Future method - rest

Question

I understand that I cannot directly call a future method from a batch class. But from many other answers, I can see that it is possible to do so by creating a helper class and calling the future method there. But it is not working for me. Please check my code below.
Also, I have tried to do it with the queueable class as suggested in this link, but it is not working for me. The error was " Callout not allowed from this future method. Please enable callout by annotating the future method. eg: #Future(callout=true)"
But I am more interested in the first and simpler way to do this.
public class OrdersItemsHelper {
static Document tDoc;
static blob csvBlob;
//prepare csv file to send
public static void CreateCsvFile(List<Order_Line_Items__c> orderItemsList)
{
//Code to create file here
csvBlob = Blob.valueOf(finalstr);
tDoc = new Document();
tDoc.Name = 'sales_items_' +date.today();
tDoc.Type = 'csv';
tDoc.body = csvBlob;
tDoc.FolderId = [select id from folder where name = 'Emarsys Order Files'].Id;
tDoc.ContentType = 'application/vnd.ms-excel';
Insert tDoc;
system.debug('doc inserted');
sendFile();
}
#Future(callout = true)
public static void sendFile()
{
System.debug('I am creating the post request');
Http http = new Http();
HttpRequest request = new HttpRequest();
request.setMethod('POST');
request.setHeader('Authorization','Security Token');
request.setHeader('Accept','text/plain');
request.setHeader('Content-Type', 'text/csv');
request.setHeader('Authorization', 'Bearer ');
request.setBodyAsBlob(csvBlob);
HttpResponse response = http.send(request);
system.debug('response: ' + response);
}

Answer 1

So I tried again by doing it in the queueable apex class. The thing I was missing was "Database.AllowsCallouts" in the class heading. Below is my queueable class which is working with the batch class to send a rest post request.
public class OrderItemFilePostHelper implements System.Queueable,Database.AllowsCallouts
{
private Blob csvBlob;
public EmarsysOrderItemFilePostHelper(Blob csvBlob) {
this.csvBlob = csvBlob;
}
public void execute(System.QueueableContext objContext)
{
System.debug('I am creating the post request');
Http http = new Http();
HttpRequest request = new HttpRequest();
request.setMethod('POST');
request.setHeader('Authorization','Security Token');
request.setHeader('Accept','text/plain');
request.setHeader('Content-Type', 'text/csv');
request.setHeader('Authorization', 'Bearer ');
request.setEndpoint('https://webhook.site/b0746268-e95c-4f94-bcb6-61d4bea54378');
request.setBodyAsBlob(csvBlob);
HttpResponse response = http.send(request);
system.debug('response: ' + response);
}
}