Sends a reply with files attached, as multipart/form-data, and marks the conversation read.
The limits are the Plum inbox's own: jpg, jpeg, png, pdf or docx, up to 20 MB each, up to 10
files. Files also pass through Plum's attachment safety checks, and a file rejected by those
is reported per file in the response rather than failing the whole request, because the
message itself is still sent. Check the Success flag on every entry.