Revert "Refactor fishAddFileFlag for better flexibility [#1156]"

This reverts commit fcce511478.

fish.go

@@ -4,7 +4,6 @@ import (
 	"bytes"
 	"fmt"
 	"io"
-	"reflect"
 	"strings"
 	"text/template"
 )
@@ -159,17 +158,24 @@ func (a *App) prepareFishFlags(flags []Flag, previousCommands []string) []string
 }
 
 func fishAddFileFlag(flag Flag, completion *strings.Builder) {
-	val := reflect.ValueOf(flag)
-	// if flag is a non-nil pointer to a struct...
-	if val.Kind() != reflect.Invalid && val.Elem().Kind() == reflect.Struct {
-		field := val.Elem().FieldByName("TakesFile")
-		// if flag's underlying type has a bool field called TakesFile, whose value is true...
-		if field.Kind() == reflect.Bool && field.Bool() {
-			// don't append '-f'
+	switch f := flag.(type) {
+	case *GenericFlag:
+		if f.TakesFile {
+			return
+		}
+	case *StringFlag:
+		if f.TakesFile {
+			return
+		}
+	case *StringSliceFlag:
+		if f.TakesFile {
+			return
+		}
+	case *PathFlag:
+		if f.TakesFile {
 			return
 		}
 	}
-	// append '-f', indicating that arguments to this flag are *not* meant to be file paths
 	completion.WriteString(" -f")
 }