diff --git a/litellm/llms/vertex_ai/gemini/transformation.py b/litellm/llms/vertex_ai/gemini/transformation.py
index f92e3ae4ca8..a109769f832 100644
--- a/litellm/llms/vertex_ai/gemini/transformation.py
+++ b/litellm/llms/vertex_ai/gemini/transformation.py
@@ -73,10 +73,13 @@ _GEMINI_MIME_TYPE_ALIASES: Dict[str, str] = {
 
 
 def _apply_gemini_mime_type_aliases(mime_type: str) -> str:
-    """Normalize known MIME aliases only; does not consult the file-type registry."""
-    return _GEMINI_MIME_TYPE_ALIASES.get(
-        mime_type.strip().lower(), mime_type.strip().lower()
-    )
+    """Normalize known MIME aliases only; does not consult the file-type registry.
+
+    Also strips MIME parameters (e.g. ``; charset=utf-8``) so that values
+    sourced from GCS object metadata (``contentType``) validate correctly.
+    """
+    normalized = mime_type.split(";", 1)[0].strip().lower()
+    return _GEMINI_MIME_TYPE_ALIASES.get(normalized, normalized)
 
 
 def _get_vertex_base() -> Any: