Where such motion to transfer venue is duly made, it shall be granted, unless the credibility of those making such application, or their means of knowledge or the truth of the facts set out in said application are attacked by the affidavit of a credible person; when thus attacked, the issue thus formed shall be tried by the judge; and the application either granted or refused. Reasonable discovery in support of, or in opposition to, the application shall be permitted, and such discovery as is relevant, including deposition testimony on file, may be attached to, or incorporated by reference in, the affidavit of a party, a witness, or an attorney who has knowledge of such discovery.