1. Open the message.
2. Ctrl + s and save it as name.mbs.
M2 Log file:
1. Enable Logging.
2. Zip up name.txt to name.txt.zip.
2. Click "choose" and browse to the zip file that was created above.
If a file with that name already exists in your file space, rename the file to something else before uploading so you don't overwrite the existing file with that name.
3. Click "Upload files".
4. Once the file is uploaded, you can right-click on its link down the page and choose "copy link address". Then, you can paste that link in a forum post.
The reason you zip up the file is to make sure it stays in tact just the way it was created by M2 (just in case the server accidentally does something funky with the bytes, which can affect analying of a problem).
The reason you don't just post the content of the file in a thread is because it can break the forum where the forum thread won't load anymore or where the forum thread takes a very long time to load, especially when the message source is really long.