I can still add them just fine, however once I have sent the changes and then try to view them again in edit-mode they are just gone (although they still exist in the post itself).
This is how my post looks like:

And this is what I see in edit-mode:

Notice how the file for 1.6.0 is still showing up because it was added before the forum update happened.
This is a problem because as you can see I wanted to replace the latest version with 1.7.0pre2, however there is no way for me to remove the old 1.7.0pre1 file. So even though I have removed it inside the post it still exists and is being shown as attachment below.
I'm not sure what to do about this.