its quite simple actually...

step 1.... scroll down till u find this in the post reply or create a new thread

[attachmentid=1218571]

2. browse ur image or file and click add this attachment

[attachmentid=1218572]

3. the more you upload, it will list down... (Tips: name the file so u can remember which is which, there is no thumbnail)

[attachmentid=1218573]

4. Click add into post button on the file manager and it will automatically inside the code with the file ID into your post

[attachmentid=1218575]

hope this help!