Sanitize file name on upload

MoneyMap/Services/ReceiptManager.cs

@@ -1,13 +1,14 @@
-using System;
-using System.IO;
-using System.Linq;
-using System.Security.Cryptography;
-using System.Threading.Tasks;
-using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.Hosting;
 using Microsoft.AspNetCore.Http;
 using Microsoft.EntityFrameworkCore;
 using MoneyMap.Data;
 using MoneyMap.Models;
+using System;
+using System.IO;
+using System.Linq;
+using System.Security.Cryptography;
+using System.Text;
+using System.Threading.Tasks;
 
 namespace MoneyMap.Services
 {
@@ -98,7 +99,7 @@ namespace MoneyMap.Services
             var receipt = new Receipt
             {
                 TransactionId = transactionId,
-                FileName = file.FileName,
+                FileName = SanitizeFileName(file.FileName),
                 StoragePath = relativeStoragePath,
                 FileSizeBytes = file.Length,
                 ContentType = file.ContentType,
@@ -112,6 +113,36 @@ namespace MoneyMap.Services
             return ReceiptUploadResult.Success(receipt);
         }
 
+        private static string SanitizeFileName(string fileName)
+        {
+            if (string.IsNullOrWhiteSpace(fileName))
+                return "receipt";
+
+            // Remove non-ASCII characters and replace them with safe equivalents
+            var sanitized = new StringBuilder();
+            foreach (var c in fileName)
+            {
+                if (c == '®' || c == '™' || c == '©')
+                {
+                    // Skip trademark/copyright symbols
+                    continue;
+                }
+                else if (c >= 32 && c <= 126)
+                {
+                    // Keep ASCII printable characters
+                    sanitized.Append(c);
+                }
+                else
+                {
+                    // Replace other non-ASCII with underscore
+                    sanitized.Append('_');
+                }
+            }
+
+            var result = sanitized.ToString().Trim();
+            return string.IsNullOrWhiteSpace(result) ? "receipt" : result;
+        }
+
         public async Task<bool> DeleteReceiptAsync(long receiptId)
         {
             var receipt = await _db.Receipts.FindAsync(receiptId);