How to Create/Delete file attachments(Content Document) through Apex ?

 There are 3 standard salesforce objects to store file attachments.

Content Document, ContentDocumentVersion, ContentDocumentLink. 

Here is the article to talk about these objects and relationship. 

https://www.forcetalks.com/blog/contentdocument-and-contentversion-in-salesforce-an-overview/

ContentDocumentVersion

ContentDocumentLink

This post is all about how to create/delete content document though Apex. Here is code snippet

// Insert Content Version record

ContentVersion contentVersionRec = new ContentVersion(Title='filename',PathOnClient ='FileName.pdf',VersionData = bodyBlob,origin = 'H');

INSERT contentVersionRec; // this will insert one record in ContentDocument and ContentVersion ,ContentDocument  is parent and ContentVersion is child record

// get contentdocument id

contentVersionRec = [SELECT Id, Title, ContentDocumentId FROM ContentVersion WHERE Id = :contentVersionRec .Id LIMIT 1];

// Create Content Document Link record- This will attach file to particular record

 ContentDocumentLink contentlink = new ContentDocumentLink();

 contentlink.LinkedEntityId = 'Sobject Id'; // for example lead or opportunity record Id

 contentlink.contentdocumentid = contentVersionRec .contentdocumentid;

 contentlink.ShareType = 'I';

 contentlink.Visibility = 'AllUsers';

 INSERT contentlink;

Note - Same content document can be shared with other objects, we just have to create ContentDocumentLink records.

Deleting File Attachments

//Delete ContentDocumentLink

Delete [select id from ContentDocumentLink where contentdocument.Title = 'ES-0275' and LinkedEntityId = '00Q1Q00001IZxcAUAT'];

// Delete Content Version 

Delete[select id from ContentVersion where Title = 'ES-0275'];

// Delete Content Document

Detete[select id from ContentDocument where Title = 'ES-0275'];

Note - If we delete content document then contentdocumentversion and contentdocumentlink will be deleted automatically.