Upload attachment with API on VBA

Hello,
I try to upload a file to a Trello card with my VBA application, but I always receive status 400 from the API.

Here is what I see in network tab of Firefox when I upload a file:

-----------------------------21770600522523212291266470825
Content-Disposition: form-data; name="dsc"

c27e21f98df1b3ed8f[...]7d49d9daf562557675dc7
-----------------------------21770600522523212291266470825
Content-Disposition: form-data; name="numOfFiles"

1
-----------------------------21770600522523212291266470825
Content-Disposition: form-data; name="fileIdx"

0
-----------------------------21770600522523212291266470825
Content-Disposition: form-data; name="file"; filename="test.txt"
Content-Type: text/plain

this is a test file

-----------------------------21770600522523212291266470825-

I tried to send a POST request with a body having the same format, but I got a status 400 (Bad request).
I have just removed the “dsc” part, I don’t know what is it or how I can calculate it. Is it mandatory ?

Query: https://api.trello.com/1/cards/[my card id]/attachments

Am I doing something wrong ?

Again, the API docs are not very useful in this case:

https://developer.atlassian.com/cloud/trello/rest/api-group-cards/#api-cards-id-attachments-post