Post a Feed Element with Existing Files

// Define the FeedItemInput object to pass to postFeedElement
ConnectApi.FeedItemInput feedItemInput = new ConnectApi.FeedItemInput();
feedItemInput.subjectId = 'me';

ConnectApi.TextSegmentInput textSegmentInput = new ConnectApi.TextSegmentInput();
textSegmentInput.text = 'Would you please review these docs?';

// The MessageBodyInput object holds the text in the post
ConnectApi.MessageBodyInput messageBodyInput = new ConnectApi.MessageBodyInput();
messageBodyInput.messageSegments = new List<ConnectApi.MessageSegmentInput>();
messageBodyInput.messageSegments.add(textSegmentInput);
feedItemInput.body = messageBodyInput;

// The FeedElementCapabilitiesInput object holds the capabilities of the feed item.
// For this feed item, we define a files capability to hold the file(s).

List<String> fileIds = new List<String>();
fileIds.add('069xx00000000QO');
fileIds.add('069xx00000000QT');
fileIds.add('069xx00000000Qn');
fileIds.add('069xx00000000Qi');
fileIds.add('069xx00000000Qd');

ConnectApi.FilesCapabilityInput filesInput = new ConnectApi.FilesCapabilityInput();
filesInput.items = new List<ConnectApi.FileIdInput>();

for (String fileId : fileIds) {
    ConnectApi.FileIdInput idInput = new ConnectApi.FileIdInput();
    idInput.id = fileId;
    filesInput.items.add(idInput);
}

ConnectApi.FeedElementCapabilitiesInput feedElementCapabilitiesInput = new ConnectApi.FeedElementCapabilitiesInput();
feedElementCapabilitiesInput.files = filesInput;

feedItemInput.capabilities = feedElementCapabilitiesInput;

// Post the feed item. 
ConnectApi.FeedElement feedElement = ConnectApi.ChatterFeeds.postFeedElement(Network.getNetworkId(), feedItemInput);