models: Update error message to reference combined MIME types

- Changed the error message to correctly list `combined_allowed_mime_types`
  instead of `allowed_mime_types` for improved clarity and accuracy
  during validation.

Assisted-by: Codex

Author: datalogics-kam

src/pdfrest/models/_internal.py

@@ -189,7 +189,10 @@ def allowed_mime_types_validator(
                 _ = allowed_mime_types_validator(item)
             return value
         if value.type not in combined_allowed_mime_types:
-            msg = error_msg or f"The file type must be one of: {allowed_mime_types}"
+            msg = (
+                error_msg
+                or f"The file type must be one of: {combined_allowed_mime_types}"
+            )
             raise ValueError(msg)
         return value