Can't use multiple display pages for list

[temuco]

I created some custom display pages for a list and when I type them in the url it changed the url to the page listed in supporting files.
for example if the default display page is

http://sharepoint/dispform.aspx?ID=1

and I change it to

http://sharepoint/dispform-custom.aspx?ID=1

and hit enter it changes the url back to

http://sharepoint/dispform.aspx?ID=1

Does anyone have any ideas? It used to work.