Currently to Reply to All a message with Attachments, it's a five step process:
1) Open the message you received
2) Click to Download Attachment
3) Click the Reply Arrow to spawn the new message
4) In the blue message header, click on the down arrow next to Reply and pick Reply to All
5) To reply with attachment, click on short Hamburguer menu, and Reply with Attachments.
then just complete your message to all of the recipients and click Send.
My suggestion is to slide up the and click on menu at bottom. See this wireframe in image