To mark the forty-eighth anniversary of the assassination of President Kennedy, the Times has posted a short documentary film by Errol Morris “The Umbrella Man.” The film features an interview with Josiah (Tink) Thompson, the author of “Six Seconds in Dallas,” a micro-study of the assassination, and centers on the mystery surrounding a man who appears in the Zapruder film, standing in Dealey Plaza, just as the shots rang out. On a perfectly pleasant day, why was this man holding a black umbrella? It was a question that launched a host of conspiracy theories related to the assassination.