Implement file upload and viewing functionality; refactor letter migrations and models to use UUIDs

app/Models/FileUpload.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Concerns\HasUuids;
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Support\Str;
+
+class FileUpload extends Model
+{
+    use HasFactory, HasUuids;
+
+    protected $table = 'file_uploads';
+    protected $keyType = 'string';
+    public $incrementing = false;
+
+    protected $fillable = ['letter_id', 'file_name', 'file_path', 'mime_type'];
+
+    protected static function boot()
+    {
+        parent::boot();
+        static::creating(function ($model) {
+            $model->id = Str::uuid()->toString();
+        });
+    }
+}