We can not find rest API for attachment.

I spoke with some of the developers regarding creating a simple HTML form that leverages the API for attachments and this is one that ChristopherB showed me for image uploads:

<form enctype="multipart/form-data" class="lia-form lia-form-horizontal" action="https://community.com/restapi/vc/messages/id/109/edit" method="post" id="form_1" name="form_1">        
        <label for="lia-image-content">Image</label>
        <input id="lia-image-content" name="message.upload" type="file" />
        <input title="" value="Upload Image" id="action.upload-image.button" name="action.upload-image" type="submit" />
</form>