feat: add MDX file support (#12261)

Feat: add MDX file support  #12057 
### What problem does this PR solve?

<img width="1055" height="270" alt="image"
src="https://github.com/user-attachments/assets/a0ab49f9-7806-41cd-8a96-f593591ab36b"
/>

The page states that MDX files are supported, but uploading fails with
the error: "x.mdx: This type of file has not been supported yet!"
<img width="381" height="110" alt="image"
src="https://github.com/user-attachments/assets/4bbb7d08-cb47-416a-95fc-bc90b90fcc39"
/>


### Type of change

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

rag/app/qa.py

@@ -421,7 +421,7 @@ def chunk(filename, binary=None, from_page=0, to_page=100000, lang="Chinese", ca
             res.append(beAdocPdf(deepcopy(doc), q, a, eng, image, poss))
         return res
 
-    elif re.search(r"\.(md|markdown)$", filename, re.IGNORECASE):
+    elif re.search(r"\.(md|markdown|mdx)$", filename, re.IGNORECASE):
         callback(0.1, "Start to parse.")
         txt = get_text(filename, binary)
         lines = txt.split("\n")