This is what I have been using successfully:
...
<div class="custom-file">
<input type="file" class="custom-file-input image-gallery" id="image-gallery" name="image-gallery[]" multiple accept="image/*">
<label class="custom-file-label" for="image-gallery">Upload Image(s)</label>
</div>
...
It is always a good idea to check for the actual file type on the server-side as well.