Error: X-Content-Type-Options: nosniff , MIME („text/html”) after using hosting

Question

Today for a first time I used hosting on infinityfree. All website works fine, but two elements from bootstrap (dropdown-menu). The error is due to MIME type ("text/html") mismatch X-Content-Type-Options: nosniff.

Can I do anything with it? I'm begginer and feeling a little lost. From what I have read I should add header in htaccess file, but on infinityfree this file is read only. Is there anything I can do or is it clearly server setting and I can't work around it?

<nav class="navbar navbar-dark navbar-expand-lg">
  <a class="navbar-brand" href="index.php">Nova Art</a>
  <button class="navbar-toggler " type="button" data-toggle="collapse" data-target="#menu" aria-controls="menu"
    aria-expanded="false" aria-label="Navigation">
    <span class="navbar-toggler-icon"></span>
  </button>

  <div class="collapse navbar-collapse" id="menu">
    <ul class="navbar-nav mr-auto">
      <li class="nav-item active"><a class="nav-link" href="index.php">Main Page</a></li>
      <li class="nav-item dropdown"><a class="nav-link dropdown-toggle" data-toggle="dropdown" role="button"
          aria-expanded="false" id="submenu" aria-haspopup="true" href="#">Categories</a>

        <div class="dropdown-menu" aria-labbeledby="submenu">

          <form class="dropdown-item" method="POST" action="category.php">
            <input class="category" type="submit" name="category" value="Photography" />
          </form>

          <form class="dropdown-item" method="POST" action="category.php">
            <input class="category" type="submit" name="category" value="Digital" />
          </form>

          <form class="dropdown-item" method="POST" action="category.php">
            <input class="category" type="submit" name="category" value="Drawings" />
          </form>

          <form class="dropdown-item" method="POST" action="category.php">
            <input class="category" type="submit" name="category" value="Paintings" />
          </form>

        </div>
      </li>
      <li class="nav-item"><a class="nav-link" href="profile.php">My profile</a></li>
      <?php
        if(isset($_SESSION['logged']) && ($_SESSION['logged']==true)){
            echo '<li class="nav-item" ><a href="loggout.php" class="nav-link " >Logout</a></li>';
        }
        else echo '<li class="nav-item" ><a class="nav-link " href="logWindow.php">Login</a></li>';
      ?>

    </ul>
    <form action="search.php" method="POST" class="form-inline">
      <input type="text" class="form-control mr-1 mt-3 logWin" placeholder="Find user" name="search" />
    </form>
  </div>
</nav>

Thanks for all the help!