Finds the first message attachment that meets a set of criteria.
It has no input but outputs a map describing the email attachment;
an individual element of the list at /request/mail/attachments
.
The output is null
if a matching attachment cannot be found.
index
attribute is
used to identify the attachment to be considered, using
/request/mail/attachments#indexValue
.Content-Type
header matches the
regular expression specified.