Introducing myself, I'm Poul from Denmark, and I'm here to learn about reliability of different media, currently DVD/Blu-ray discs in particular.
First off, MyCE appears to be a solid community with plenty of valuable info. But I will question your policy about restricting access to attachments. From my point of view it serves as "bait" to make new members. If I'm wrong, enlighten me about the actual purpose.
Regardless, in practice this is how it works. You gain new members who join not because they can/will contribute new info, but because they cannot access the info they're looking for in any other way. Such members will then be disappointed/annoyed to find that they still can't view attachments after joining. Some will leave angry and never come back. Others will make 2 posts in order to access attachments even though they have nothing useful to post. Finally, some do start making meaningful contributions, but given that they actually have something to contribute there's a fair chance they'd eventually join without "bait" anyway - and that they would have been happier to do so.
The Internet is full of meaningless and/or dead memberships and clutter that makes it harder to find the info you are looking for. Like it or not, your attachment policy contributes to this, and it somewhat ruins the first impression of MyCE. It's not good for the Internet, and it's hard to believe that it is actually good for MyCE either - other than for the membership number, but I'd argue that's a poor metric for the quality of a user community.
Whether the attachment policy really is bait or it serves a more noble purpose I would urge you to consider if it is really the best option. I hope you can agree that it has some highly undesirable drawbacks. This was the only way I could think of making a meaningful contribution, but I would much rather have been able to see the info I came for and saved us the time.