The Content tables are considerably more complex than Attachments. The file content is held in the
ContentVersion object; one
ContentDocument has one or many versions, as well as an arbitrary number of
ContentDocumentLink records connecting it to records to which it is shared.
Supposing that you already have the Id of a
ContentDocument attached to some record or another, you can get the relevant file content in SOQL:
SELECT VersionData FROM ContentVersion WHERE ContentDocumentId = :myId AND IsLatest = true
ContentVersion object reference may be useful.
If you need to acquire files that are attached to a specific object in Salesforce, you'd need to first perform a query against
ContentDocumentLink and filter upon
LinkedEntityId with the Id of the record-attached-to. The
ContentDocumentId will then let you perform the above query against