Under that circumstance (.7z files with renamed extension) I can confirm that it is NOT possible to copy a file into the archive if it is opened with Enter, but it is when it is possible when opened with Ctrl+PgDn.I meant renamed 7z archives. Let's say a .cb7. If it's not associated with anything pressing enter will open it and show the files just like when the extension is 7z, even though TC doesn't show it with an archive icon. But when I press F5, I get the unsupported function.
I can however copy files from the archive to a folder even if the archive is opened with Enter.
Also I cannot delete files in the archive if I have opened it with Enter, just as I cannot delete files in the archive if I have disable the Total7zip plugin.
So the difference seems to be that when pressing Enter on the file It is TC's own TC7Z64.DLL that exclusively handles the archive - and TC7Z64.DLL can only open archives and extract files from archives. When pressing Ctrl+PgDn the Total7zip plugin is handeling copying to the archive, and deleting files in the archive - hence no error message. When pressing Ctrl+PgDn I don't know whether it is Total7zip plugin that opens the archive or copies files from it or that is done by TC7Z64.DLL.
In contrast to Total7zip that apparently handles archives with unknown filetypes when pressing Ctrl+PgDn, it seems like the 7zip plugin only handles (extracting and deleting) archives with extensions that it is registered to in wincmd.ini. This leads me to believe that it is always TC7Z64.DLL that opens unknown filetypes, when pressing Ctrl+PgDn.
My 7zip.wcx plugin is registered with the parameter 223 and my Total7zip.wcx plugin is registered vith parameter 735, and that means that both plugins should be able to "Detect archive type by content", but apparently 7zip does not do so.