Initial version

DataTransfer.Client/Services/FileNameSanitizer.cs

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+using System.Globalization;
+using System.Text;
+
+namespace DataTransfer.Client.Services;
+
+public static class FileNameSanitizer
+{
+    private const int MaxLength = 255;
+    private const string Fallback = "file";
+
+    // Characters invalid in file names on Windows, macOS or Linux. Path.GetInvalidFileNameChars() returns only the
+    // Unix set in WebAssembly, but the file is saved on any operating system.
+    private const string InvalidCharacters = "<>:\"/\\|?*";
+
+    /// <summary>
+    ///     Makes a file name received from the peer safe to display and save. Invisible formatting characters are removed,
+    ///     so for example a right-to-left override can't disguise <c>invoice&#x202E;fdp.exe</c> as <c>invoiceexe.pdf</c>.
+    /// </summary>
+    public static string Sanitize(string? name)
+    {
+        if (string.IsNullOrEmpty(name))
+        {
+            return Fallback;
+        }
+
+        var builder = new StringBuilder(Math.Min(name.Length, MaxLength));
+        foreach (var rune in name.EnumerateRunes())
+        {
+            switch (Rune.GetUnicodeCategory(rune))
+            {
+                case UnicodeCategory.Format:
+                    continue;
+                case UnicodeCategory.Control:
+                    Append(builder, new Rune('_'));
+                    continue;
+            }
+
+            Append(builder, rune.IsAscii && InvalidCharacters.Contains((char)rune.Value) ? new Rune('_') : rune);
+        }
+
+        // Windows silently drops trailing dots and spaces, which could change the extension.
+        var cleaned = builder.ToString().Trim().TrimEnd('.');
+        return cleaned.Length == 0 ? Fallback : cleaned;
+    }
+
+    // Appends whole runes only, so truncation never splits a surrogate pair.
+    private static void Append(StringBuilder builder, Rune rune)
+    {
+        if (builder.Length + rune.Utf16SequenceLength <= MaxLength)
+        {
+            builder.Append(rune.ToString());
+        }
+    }
+}