When jed prompts for a file name or a buffer name, the space bar and the TAB keys are special. Hitting the TAB key will complete the name that is currently in the minibuffer up until it is no longer unique. At that point, you can either enter more characters to complete the name or hit the space bar to cycle among the possible completions. The spacebar must be pressed at least twice to cycle among the completions.
On MSDOS and VMS, it is possible to use wildcard characters in the file name for completion purposes. For example, entering *.C and hitting the space bar will cycle among file names matching *.C. Unfortunately, this feature is not available on unix systems.