Hi Pearce, documentation of uploading files via the API is a bit scattered, but I was able to get get it to work a while back. Here are some links that should help you out.
https://api.developer.monday.com/docs/files-queries
A couple tips… When uploading files, you should use an alternate endpoint: https://api.monday.com/v2/file
Also remember that will need to create a multipart request. There are some tips in the links I posted for this.
Here is an example of a curl request:
curl --location --request POST 'https://api.monday.com/v2/file' \
--header 'Authorization: yourSuperSecretApiKey' \
--form 'query="mutation add_file($file: File!, $update_id: Int!) {add_file_to_update(update_id: $update_id, file: $file) {id}}"' \
--form 'variables="{\"update_id\":920768626}"' \
--form 'map="{\"image\":\"variables.file\"}"' \
--form 'image=@"path to your file/in local memory/cool picture.jpg'