Below the reply box click on Go Advanced.Then scroll down until you see a button Manage Attachments. Click on that and a popup-window opens.
Click on the Browse button, find the minidump files you wish to attach on your PC and doubleclick on it. Normally found in C:\Windows\minidump
Now click on the Upload button in the popup. When done, click on the Close this window button.
Enter your message-text, then click on Submit Message.