Since I see a lot of people have trouble upl0ading their hacks and stuff to the forum I decided to make a tutorial.
step 1: You type here what you want to type.
step 2: You scroll down thill you see the box named "Additional Options" (first picture of the attachment)
step 3: After you found the box click on "Manage Attachments" (A)
step 4: A window will pop up (second picture) which shows the option to
upload a file file from your computer (B) and an option to upload a file drom an URL(C). Underneath is attachment key.
step 5: If you want to upload a file from your computer select the browse button (B). Look for your file and click "open". Then click upload.
If you want to upload a file from an URL you enter the URL in the box beneath (C) and hit the upload button.
step 6: If everything went allright your attachment will be shown underneath the header "Current Attachments" (picture 3)
step 7: If that is the item you wanted to upload click "close this window" in the top right corner. If not remove the attachment and upload the right one.
step 8: click "Submit New Thread" and there you go you added an attachment to your thread.
Hope this was clear enough to understand, good luck.
