What attachment file formats supported with support system plugin ?

I had some users report that their attachments were not showing when submitting a ticket.
What file formats are supported? So I was testing with images, pdf's & word docs. I noticed that word docs don't submit and there is no error message of any kind so you don't that it didn't attach or what the reason would be so that can cause confusion? It just submits the ticket without the attachment.
thanks!