added uniqueFileName rule

app/Rules/UniqueFileName.php

@@ -0,0 +1,51 @@
+<?php
+
+namespace App\Rules;
+
+use Illuminate\Contracts\Validation\Rule;
+use Illuminate\Support\Facades\Config;
+use Illuminate\Support\Facades\Storage;
+
+class UniqueFileName implements Rule
+{
+    /**
+     * Create a new rule instance.
+     *
+     * @return void
+     */
+    public function __construct()
+    {
+        //
+    }
+
+    /**
+     * Determine if the validation rule passes.
+     *
+     * @param  string $attribute
+     * @param  mixed  $value
+     * @return boolean
+     */
+    public function passes($attribute, $value)
+    {
+        // Get the list of storage disks from the filesystems configuration file
+        $disks = array_keys(Config::get('filesystems.disks'));
+
+        foreach ($disks as $disk) {
+            if (Storage::disk($disk)->exists($value) === true) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+
+    /**
+     * Get the validation error message.
+     *
+     * @return string
+     */
+    public function message()
+    {
+        return 'The file name already exists.';
+    }
+}