You'll need a custom page. Read the Add a New Custom Page to phpBB
Knowledge Base article. On that page, put code in similar to how attachments are displayed in a post, but do that for all of the files in the attachments
table. You'll also probably want to ensure that the list only contains files that people have permission to see.
If you want the list to be sortable, you could look at the Member List code. I would have columns for file name, file size, date, uploader (post author) and original location (a link to the post including the file).
P.S. Please try to use better subjects. Many people ignore content-free subjects like "New User" or "Please Help". Something like "Need a Page to Show All Uploaded Attachments" would be better.