code: public @NotNull FileAssert isFile() {
      isNotNull();
      if (actual.isFile()) {
        return this;
      }
      failIfCustomMessageIsSet();
      throw failure(format("file:<%s> should be a file", actual));
    }
docs: Verifies that the actual {@code File} is a regular file.
  
  @return this assertion object.
  @throws AssertionError if the the actual {@code File} is {@code null}.
  @throws AssertionError if the actual {@code File} is not a regular file.