Tell me please, what is the main difference between options CURLOPT_MIMEPOST and CURLOPT_POSTFIELDS ?
What can be done with a CURLOPT_MIMEPOST - that cannot be done with CURLOPT_POSTFIELDS ?
Sends exactly the bytes you specify in the body of the HTTP request. With a default Content-type of application/x-www-form-urlencoded. libcurl will not add or encode the data in any way for you.
With the curl command line tool, you do this with -d.
Makes libcurl send a "multipart formpost". That is a data stream using a format that allows the sender to send multiple "parts" of data to the server, each part being properly separated and identified. Each part has a name, content and its own set of headers. When an HTTP client "uploads a file", this is almost always done using multipart formposts.
Multipart formpost is structured data in the request body and this option helps you produce and send that format. An application can also produce that format by themselves should they prefer that and provide it with CURLOPT_POSTFIELDS or even using the callback CURLOPT_READFUNCTION.
With the curl command line tool, you do this with -F.