community: add init for unstructured file loader (#29101)

## Description
Add `__init__` for unstructured loader of
epub/image/markdown/pdf/ppt/word to restrict the input type to `str` or
`Path`.
In the
[signature](https://python.langchain.com/api_reference/community/document_loaders/langchain_community.document_loaders.markdown.UnstructuredMarkdownLoader.html)
these unstructured loaders receive `file_path: str | List[str] | Path |
List[Path]`, but actually they only receive `str` or `Path`.

## Issue
None

## Dependencies
No changes.

libs/community/langchain_community/document_loaders/excel.py

@@ -42,6 +42,7 @@ class UnstructuredExcelLoader(UnstructuredFileLoader):
               for more info. Optional. Defaults to "single".
             **unstructured_kwargs: Keyword arguments to pass to unstructured.
         """
+        file_path = str(file_path)
         validate_unstructured_version(min_unstructured_version="0.6.7")
         super().__init__(file_path=file_path, mode=mode, **unstructured_kwargs)