code: public void assertEqualContent(Description description, File actual, File expected, Charset charset) {
      checkIsFile(expected);
      checkNotNull(charset);
      assertIsFile(description, actual);
      try {
        String lineSeparator = System.getProperty("line.separator");
        FileDiffs diffs = comparator.compareContents(actual, expected, charset);
        if (!diffs.isEmpty()) {
          String format = "files expected:<%s> and actual:<%s> do not have equal content:" + lineSeparator + "%s"
              + lineSeparator + "using charset:<%s>";
          throw failures.failure(description, new BasicErrorMessageFactory(format, expected, actual, diffs, charset));
        }
      } catch (IOException e) {
        String format = "Failed to compare contents of files:<%s> and:<%s> using charset:<%s>";
        throw new IORuntimeException(String.format(format, actual, expected, charset), e);
      }
    }
docs: Asserts that the given {@code File}s represent existing files with equal content. It is assumed that the content of
  the files is text.
  
  @param description the description of the <em>actual</em> {@code File}.
  @param actual the <em>actual</em> {@code File}.
  @param expected the <em>expected</em> {@code File}.
  @param charset the charset to use when reading the contents of the given {@code File}s.
  @throws NullPointerException if the <em>expected</em> {@code File} is {@code null}.
  @throws IllegalArgumentException if the <em>expected</em> {@code File} does not represent an existing file.
  @throws NullPointerException if the given charset is {@code null}.
  @throws AssertionError if the <em>actual</em> {@code File} is {@code null}.
  @throws AssertionError if the <em>actual</em> {@code File} does not represent an existing file.
  @throws IORuntimeException if an I/O error occurs.
  @throws AssertionError if the given files do not have equal content.