Slack API - Attatchments from custom bot post as plain text

As it turns out, the call to

slack_client.api_call("chat.postMessage", channel=self.channel, attachments=self.msg, as_user=True)

apears to add the top layer { "attachments": ... } for you. So by changing my self.msg to simply be:

self.format = [{
    "fallback": "%s, %s" % (self.jiraIssueObj.fields.summary, self.link),
    "pretext": "Detail summary for %s" % self.jiraIssueObj,
    "title": self.jiraIssueObj.fields.summary,
    "title_link": self.link,
    "text": self.jiraIssueObj.fields.description[0:self.maxSummary],
    #"color": "#7CD197",
    "mrkdwn_in": ["text", "pretext", "fields"]
}]

without this outer { "attachments": ... } wrapper, the api was able to post the message attachment as expected.