Welcome to the Monday Community.
Unfortunately the limitation here is that emails sent from Monday currently can only be plain text. This means no attachments or hyper links. If you get the share link from Dropbox, you can put the text for the link into a Text column which will work with the email integrations. But note that it will not generate a clickable link in the email, your client would need to copy & paste.
More robust email features are in the works for Monday, but right now the only way to remedy this is to use a 3rd party integration app like Integromat & Zapier. Both are “low code” options and I’ve personally used Integromat to do exactly what you are asking for and then some.
Hope this helps!