curl_mime_free man page
curl_mime_free — free a previously built mime structure
void curl_mime_free(curl_mime *mime);
curl_mime_free(3) is used to clean up data previously built/appended with curl_mime_addpart(3) and other mime-handling functions. This must be called when the data has been used, which typically means after curl_easy_perform(3) has been called.
The handle to free is the one you passed to the CURLOPT_MIMEPOST(3) option: attached subparts mime structures must not be explicitly freed as they are by the top structure freeing.
mime is the handle as returned from a previous call to curl_mime_init(3) and may be NULL.
As long as at least one of HTTP, SMTP or IMAP is enabled. Added in 7.56.0.
curl_formfree(3), curl_mime_init(3), libcurl-tutorial(3).