Mohun Biswas clarified

docs/libcurl/curl_formadd.3

@@ -106,8 +106,9 @@ long which gives the length of the buffer.
 Tells libcurl to use the \fICURLOPT_READFUNCTION\fP callback to get data. The
 parameter you pass to \fICURLFORM_STREAM\fP is the pointer passed on to the
 read callback's fourth argument. If you want the part to look like a file
-upload one, set the \fICURLFORM_FILENAME\fP parameter as well. (Option added
-in libcurl 7.18.2)
+upload one, set the \fICURLFORM_FILENAME\fP parameter as well. Note that when
+using \fICURLFORM_STREAM\fP, \fICURLFORM_CONTENTSLENGTH\fP must also be set
+with the total expected length of the part. (Option added in libcurl 7.18.2)
 .IP CURLFORM_ARRAY
 Another possibility to send options to curl_formadd() is the
 \fBCURLFORM_ARRAY\fP option, that passes a struct curl_forms array pointer as