Description:
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The documentation page on curl_setopt does not clearly state how to select form content type (application/x-www-form-urlencoded or multipart) and how to upload a file when making a POST. I could find it out from the user notes below the manual. It seems that the multipart mode is triggered when the CURLOPT_POSTFIELDS parameter is an array, not a string, and to pass a file one has to put it like "@filename". User notes mention it, while the manual itself does not. Also, it is not clear (if possible) how to send a form in multipart mode when the file is empty (sometimes that's needed).