Washington - The FBI opened its Frank Sinatra files yesterday, a four-decade dossier featuring mob bosses Sam Giancana and Lucky Luciano. Going back to Sinatra's 1938 arrest on a seduction charge in Hackensack, New Jersey - a grand jury refused to indict him - the 1,275 pages of documents paint a picture of an agency with infinite appetite for information on Sinatra, who died in May at the age of 82. For all their record-keeping, the FBI's personnel never proved Sinatra was directly involved in the Mafia or the Communist Party. The dossier includes fuzzy black-and-white pictures of Sinatra with Jacqueline Kennedy