Upload attachment with API on VBA

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:

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

Content-Disposition: form-data; name="numOfFiles"

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

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

this is a test file


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: