DirectoryReader: set text/plain as default mime type if it is not found (#513)

DirectoryReader: set text/plain as default mime type if it is not found.

This is useful when parsing several markdown formats that don't have a mime type.

Author: NikolaosPapailiou

apis/python/src/tiledb/vector_search/object_readers/directory_reader.py

@@ -385,6 +385,9 @@ def lazy_load(
             mime_type = mimetypes.guess_type(self.uri)[0]
             f = vfs.open(self.uri)
 
+        if mime_type is None:
+            mime_type = "text/plain"
+
         if mime_type.startswith("image/"):
             from langchain_community.document_loaders import UnstructuredFileIOLoader