how to send message with image to slack channel using java
"attachments": [{
"fallback": "The attachement isn't supported.",
"title": "VALENTINE'S DAY OFFER",
"color": "#9C1A22",
"pretext": "Today's list of awesome offers picked for you",
"author_name": "Preethi",
"author_link": "https://www.hongkiat.com/blog/author/preethi/",
"author_icon": "https://assets.hongkiat.com/uploads/author/preethi.jpg",
"mrkdwn_in": ["text","fields"],
"text": "Just click the site names and start buying. Get *extra reduction with the offer code*, if provided.",
"thumb_url": "http://example.com/thumbnail.jpg"
}]