from mpart import mpart
  from mpart import boundary
  body=mpart()
  params = urlencode({
    'auth_nick': get_auth_nickname(),
    'auth_token': get_auth_token(),
    'l' : 'discussion',
    'l2' : 'send',
    'id' : p_disc_key,
    'message' : p_message,
    'attachment' : body
  })  
  req=urllib.request.Request(url)
  req.add_header('content-disposition' , 'attachment')
  req.add_header('content-type' , 'multipart/form-data; boundary="' + boundary + '"')
  req.add_header('content-length' , str(len(params)))
  f=urllib.request.urlopen(req,data=params.encode('utf8'))
  print(f.read())

<a href="http://google.com"></a> <a href="http://i.mg/t.jpg"><img src="http://nyx.cz/...../thumb.jpg"></a>

<a href="http://google.com"><img src="http://i.mg/t.jpg"></a>

<a href="http://google.com"><a href="http://i.mg/t.jpg"><img src="http://nyx.cz/...../thumb.jpg"></a></a>

curl --location --request POST 'http://www.nyx.cz/api.php' \
--form 'auth_nick=***' \
--form 'auth_token=***' \
--form 'l=discussion' \
--form 'l2=messages' \
--form 'id=14158' \
--form 'direction=older' \
--form 'id_wu=51989987'

{
    "attributes": [],
    "data": [
        {
            "id_wu": "51989986",
            "nick": "ALENNAI",
            "time": "1588769724",
            "content": "Letenské náměstí, Pha<br/>\r\n1906<br/>\r\n<a href=\"https://scontent-frt3-1.xx.fbcdn.net/v/t1.0-9/95868467_10222313885457008_571386579155681280_n.jpg?_nc_cat=107&_nc_sid=1480c5&_nc_ohc=nvnC0EI1qMAAX-2mtaB&_nc_ht=scontent-frt3-1.xx&oh=7d89cd186d79edc4ef2375e70a80b7bc&oe=5ED82A68\"><img src=\"http://www.nyx.cz/i/t/57e4561116d3732129579027e75c25f2.png?url=https%3A%2F%2Fscontent-frt3-1.xx.fbcdn.net%2Fv%2Ft1.0-9%2F95868467_10222313885457008_571386579155681280_n.jpg%3F_nc_cat%3D107%26_nc_sid%3D1480c5%26_nc_ohc%3DnvnC0EI1qMAAX-2mtaB%26_nc_ht%3Dscontent-frt3-1.xx%26oh%3D7d89cd186d79edc4ef2375e70a80b7bc%26oe%3D5ED82A68\" class=\"thumb\"></a><br/>\r\nsoučasnost<br/>\r\n<a href=\"https://upload.wikimedia.org/wikipedia/commons/4/4a/Letensk%C3%A9_n%C3%A1m%C4%9Bst%C3%AD%2C_od_z%C3%A1padu%2C_tramvajov%C3%A1_zast%C3%A1vka.jpg\"><img src=\"http://www.nyx.cz/i/t/a9afd1f6e5b05fcdaa9ceb41840dfaf2.png?url=https%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2F4%2F4a%2FLetensk%25C3%25A9_n%25C3%25A1m%25C4%259Bst%25C3%25AD%252C_od_z%25C3%25A1padu%252C_tramvajov%25C3%25A1_zast%25C3%25A1vka.jpg\" class=\"thumb\"></a> ",
            "wu_rating": "47",
            "wu_type": "0"
        },
...

Letenské náměstí, Pha<br/> 1906<br/> <img src="https://scontent-frt3-1.xx.fbcdn.net/v/t1.0-9/95868467_10222313885457008_571386579155681280_n.jpg?_nc_cat=107&amp;_nc_sid=1480c5&amp;_nc_ohc=nvnC0EI1qMAAX-2mtaB&amp;_nc_ht=scontent-frt3-1.xx&amp;oh=7d89cd186d79edc4ef2375e70a80b7bc&amp;oe=5ED82A68"><br/> současnost<br/> <img src="https://upload.wikimedia.org/wikipedia/commons/4/4a/Letensk%C3%A9_n%C3%A1m%C4%9Bst%C3%AD%2C_od_z%C3%A1padu%2C_tramvajov%C3%A1_zast%C3%A1vka.jpg">