Create Matcher to validate mime-types

[mmichaelis]

Add a matcher for validating mime-types, e. g. (here: inner class):

  private static class IsValidMimeType extends CustomTypeSafeMatcher<String> {
    private final String defaultMimeType;

    private IsValidMimeType(final String defaultMimeType) {
      super("valid mimetype");
      this.defaultMimeType = defaultMimeType;
    }

    @Override
    protected void describeMismatchSafely(final String item, final Description mismatchDescription) {
      super.describeMismatchSafely(item, mismatchDescription);
      mismatchDescription.appendText("raising ");
      try {
        new MimeType(item);
      } catch (MimeTypeParseException e) {
        mismatchDescription.appendValue(e);
      }
    }

    @Override
    protected boolean matchesSafely(final String item) {
      try {
        new MimeType(defaultMimeType);
      } catch (MimeTypeParseException ignored) {
        return false;
      }
      return true;
    }
  }