Remove client MIME type parameter from Upload

- Fix: Always detect MIME type server-side from file content instead of
  trusting or forwarding the client-supplied value.

internal/service/file.go

@@ -60,7 +60,8 @@ func NewFileService(
 }
 
 // Upload stores a file's content and creates its metadata record.
-func (s *FileService) Upload(ctx context.Context, userID string, parentID *string, fileName string, reader io.Reader, clientMime string) (*FileInfo, error) {
+// The MIME type is always detected server-side from the file content.
+func (s *FileService) Upload(ctx context.Context, userID string, parentID *string, fileName string, reader io.Reader) (*FileInfo, error) {
 	if err := validateFileName(fileName); err != nil {
 		return nil, err
 	}
@@ -83,19 +84,15 @@ func (s *FileService) Upload(ctx context.Context, userID string, parentID *strin
 		reader = io.LimitReader(reader, s.maxUploadSize+1)
 	}
 
-	// Detect MIME type when the client does not provide a meaningful one.
-	mimeType := clientMime
-	if mimeType == "" || mimeType == "application/octet-stream" {
-		// Read the first 512 bytes for detection, then replay them.
-		head := make([]byte, 512)
-		n, readErr := io.ReadFull(reader, head)
-		if readErr != nil && readErr != io.ErrUnexpectedEOF && readErr != io.EOF {
-			return nil, fmt.Errorf("read for mime detection: %w", readErr)
-		}
-		head = head[:n]
-		mimeType = mimetype.Detect(head).String()
-		reader = io.MultiReader(bytes.NewReader(head), reader)
+	// Detect MIME type from file content.
+	head := make([]byte, 512)
+	n, readErr := io.ReadFull(reader, head)
+	if readErr != nil && readErr != io.ErrUnexpectedEOF && readErr != io.EOF {
+		return nil, fmt.Errorf("read for mime detection: %w", readErr)
 	}
+	head = head[:n]
+	mimeType := mimetype.Detect(head).String()
+	reader = io.MultiReader(bytes.NewReader(head), reader)
 
 	fileID := uuid.NewString()
 	storagePath := fmt.Sprintf("%s/%s", userID, fileID)