While working on #1211 I noticed another bug. The zsh completion script performs file completion only if it receives no completions. Notice the first line of:

However, it is valid for ValidArgsFunction to return completions that don't apply to the current toComplete prefix. This means file completion should be performed, but the script does not realize it.

Say I have the following ValidArgsFunction for command withfile:

ValidArgsFunction: func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
    // We do not consider the toComplete prefix and always return a completion
    return []string{"why"}, cobra.ShellCompDirectiveDefault
},

If I have a file called myfile and run in zsh:

$ ./testprog withfile my<TAB>

I would expect to get file completion and get myfile as a completion, but I don't. This is because

$ ./testprog __complete withfile my<ENTER>
why
:0
Completion ended with directive: ShellCompDirectiveDefault

Notice that above, the script would receive a completion even though it is not technically valid. This is allowed. But then, the script does not realize it must do file completion.

I will post a PR to handle this better.