Add explicit markdown parser for DirectoryTextReader (#566)

Author: NikolaosPapailiou

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

@@ -367,6 +367,7 @@ def __init__(
             handlers={
                 "application/pdf": PyMuPDFParser(),
                 "text/plain": TextParser(),
+                "text/markdown": TextParser(),
                 "text/html": BS4HTMLParser(),
                 "application/msword": MsWordParser(),
                 "application/vnd.openxmlformats-officedocument.wordprocessingml.document": (
@@ -397,7 +398,7 @@ def lazy_load(
             mime_type = mimetypes.guess_type(self.uri)[0]
             f = vfs.open(self.uri)
 
-        if mime_type is None:
+        if mime_type is None or mime_type.startswith("text"):
             mime_type = "text/plain"
 
         if mime_type.startswith("image/"):