code: public static HttpFileBuilder of(HttpData data, long lastModifiedMillis) {
          requireNonNull(data, "data");
          return new HttpDataFileBuilder(data, lastModifiedMillis)
                  .autoDetectedContentType(false); // Can't auto-detect because there's no path or URI.
      }
docs: Returns a new {@link HttpFileBuilder} that builds an {@link AggregatedHttpFile} from the specified
  {@link HttpData} and {@code lastModifiedMillis}. Note that the {@link #build()} method of the returned
  builder will always return an {@link AggregatedHttpFile}, which guarantees a safe downcast:
  <pre>{@code
  AggregatedHttpFile f = (AggregatedHttpFile) HttpFileBuilder.of(HttpData.ofUtf8("foo"), 1546923055020)
  .build();
  }</pre>
  
  @param data the content of the file
  @param lastModifiedMillis the last modified time represented as the number of milliseconds
  since the epoch