Reading your question, I think that you have done most the steps.
But to make it sure, I did the following steps to create a discussion with an attachment.
(1) Created a project
(2) To make the size of attachment, clean the project
Select the project from the Workspace Explorer, right-click the project and select "Clean <project>"
Menu: Build > Clean <project>
(3) Create archive file of the project
From the Workspace Explorer, right-click the project then select "Archive Workspace/Project..."
After selecting the project in the Workspace Explorer, Menu: Project > Archive Workspace/Project...
(4) Workspace/Project Archive dialog appears, check "Compress Archive" and "Minimal"
(I think that they are defaults)
Then select [Archive]
(5) Summary Dialog appears, select [OK]
(6) The folder containing the archive file opened.
Note the path or keep the folder opened for later reference.
(7) Start (create) a discussion in the forum of your choice.
(8) To create a discussion, enter "Title", check "Mark this discussion as a question", if it is so.
Then enter the content of the discussion (or question)
Then select "Attache" at the right bottom.
(9) A file select dialog shows up.
Select the archive file, you created at the step (6) then select [Open] (if in Japanese [開く(O)])
(10) Now the archive is attached
(11) Then post the discussion using the left bottom button [Post]
(12) A discussion (question) with an attachment was created.
Hope this can be a hint for you ;-)
Some additional tips.
During the discussion, the default reply window does not have "Attach" button.
Then select "Use advanced editor" in the top right corner
The "Advanced" Editor has "Attach" in the bottom right corner.
In case you want to attache a file to the original post of yours, although there is no "Edit" button in the bottom left,
there is "Edit" in the right side of the message.