Enhance FileAttachment struct with existence check and file type determination

app/MindWork AI Studio/Chat/FileAttachment.cs

@@ -1,6 +1,57 @@
+using AIStudio.Tools.Rust;
+
 namespace AIStudio.Chat;
 
 /// <summary>
 /// Represents an immutable file attachment with details about its type, name, path, and size.
 /// </summary>
-public readonly record struct FileAttachment(FileAttachmentType Type, string FileName, string FilePath, long FileSize);
+public readonly record struct FileAttachment(FileAttachmentType Type, string FileName, string FilePath, long FileSizeBytes)
+{
+    /// <summary>
+    /// Gets a value indicating whether the file still exists on the file system.`
+    /// </summary>
+    public bool Exists => File.Exists(this.FilePath);
+
+    /// <summary>
+    /// Creates a FileAttachment from a file path by automatically determining the type,
+    /// extracting the filename, and reading the file size.
+    /// </summary>
+    /// <param name="filePath">The full path to the file.</param>
+    /// <returns>A FileAttachment instance with populated properties.</returns>
+    public static FileAttachment FromPath(string filePath)
+    {
+        var fileName = Path.GetFileName(filePath);
+        var fileSize = File.Exists(filePath) ? new FileInfo(filePath).Length : 0;
+        var type = DetermineFileType(filePath);
+
+        return new FileAttachment(type, fileName, filePath, fileSize);
+    }
+
+    /// <summary>
+    /// Determines the file attachment type based on the file extension.
+    /// Uses centrally defined file type filters from <see cref="FileTypeFilter"/>.
+    /// </summary>
+    /// <param name="filePath">The file path to analyze.</param>
+    /// <returns>The corresponding FileAttachmentType.</returns>
+    private static FileAttachmentType DetermineFileType(string filePath)
+    {
+        var extension = Path.GetExtension(filePath).TrimStart('.').ToLowerInvariant();
+
+        // Check if it's an image file:
+        if (FileTypeFilter.AllImages.FilterExtensions.Contains(extension))
+            return FileAttachmentType.IMAGE;
+
+        // Check if it's an audio file:
+        if (FileTypeFilter.AllAudio.FilterExtensions.Contains(extension))
+            return FileAttachmentType.AUDIO;
+
+        // Check if it's an allowed document file (PDF, Text, or Office):
+        if (FileTypeFilter.PDF.FilterExtensions.Contains(extension) ||
+            FileTypeFilter.Text.FilterExtensions.Contains(extension) ||
+            FileTypeFilter.AllOffice.FilterExtensions.Contains(extension))
+            return FileAttachmentType.DOCUMENT;
+
+        // All other file types are forbidden:
+        return FileAttachmentType.FORBIDDEN;
+    }
+}