add support for markdown file in one parse way (#2052)

### What problem does this PR solve?

#2021 add support for markdown file in one parse way

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

Co-authored-by: Zhedong Cen <cenzhedong2@126.com>

rag/app/one.py

         excel_parser = ExcelParser()
         sections = excel_parser.html(binary, 1000000000)
 
-    elif re.search(r"\.txt$", filename, re.IGNORECASE):
+    elif re.search(r"\.(txt|md|markdown)$", filename, re.IGNORECASE):
         callback(0.1, "Start to parse.")
         txt = ""
         if binary: