file
Sending files using webhooks is only possible using Content-Type: multipart/form-data header. Using this method also means you have to set json body as value of payload_json parameter.
Parameter names should have unique names otherwise they will collide and only first file from ones with identical names will be shown.
Example:
file1=@cat.jpg
file2=@dog.jpg
payload_json={"embeds":[{"title":"test"}]}
Embedding attachments
To put image attachment inside embed use attachment:// with its filename (not field name). That will also hide attachment from the message.
file1=@pizza.jpg
payload_json={"embeds":[{"image":{"url":"attachment://pizza.jpg"}}]}