Attachment myAttachment =[SELECT Id, Name,Body,Description,ContentType FROM  Attachment where Id =:myFile.Attachment_ID__c];

String communityId = 'myCommunityId';

// CREATE A FEED_ITEM_TEXT
ConnectApi.MessageBodyInput messageInput = new ConnectApi.MessageBodyInput();
messageInput.messageSegments = new List<ConnectApi.MessageSegmentInput>();

ConnectApi.TextSegmentInput textSegment = new ConnectApi.TextSegmentInput();
textSegment.text = shareComment;

messageInput.messageSegments.add(textSegment);

ConnectApi.FeedItemInput input = new ConnectApi.FeedItemInput();
input.body = messageInput;

// POST FILE_FEED_ITEM
ConnectApi.NewFileAttachmentInput fileIn = new ConnectApi.NewFileAttachmentInput();
fileIn.title = fetchDigitalChatterFile.Attachment_Name__c;
input.attachment = fileIn;

Blob userFile = myAttachment.Body;

ConnectApi.BinaryInput feedBinary = new ConnectApi.BinaryInput(userFile,myAttachment.ContentType,myAttachment.name);

// PARAMETERS(communityId, feedType, subjectId, input,filebody);
ConnectApi.FeedItem feadItem = ConnectApi.ChatterFeeds.postFeedItem(communityId, ConnectApi.FeedType.Record,'share To Id',input,feedBinary);