[Feature Request]: HttpPost / fixed multipart name

### What would you like to happen?

The httppost transfomer can be configured to send a request with a multipart.
Currently, the multipart name is hardcoded as "file":

The feature request is about make it configurable.

### Issue Priority

Priority: 3

### Issue Component

Component: Transforms

Start at the HttpPost transform configuration and locate where the multipart name is hardcoded as "file". Make that name configurable for multipart requests, then verify that the transform sends the configured name instead of the default.

Written by the indexing model from the issue text.