Let's say you upload a file under any record or chatter, the file reference is retrieved from
ContentDocument represents the library in Salesforce CRM Content or Salesforce Files.
Based on the ContentDocumentId, you can retrieve the ShareType and LinkedEntityId from
SELECT Id, ShareType, LinkedEntityId, Visibility
WHERE ContentDocumentId = '069q00000001jv0'
LinkedEntityId will return list of users.
Required. The permission granted to the user of the shared file in a library. This is determined by the permission the user already has in the library. This field is available in API version 25.0 and later.
Viewer premission. The user can explicitly view but not edit the shared file.
Collaborator permission. The user can explicitly view and edit the shared file.
Inferred permission. The user’s permission is determined by the related record. For shares with a library, this is defined by the permissions the user has in that library.
ID of the linked object. Can include Chatter users, groups, records (any that support Chatter feed tracking including custom objects), and Salesforce CRM Content libraries.
Specifies whether this file is available to all users, internal users, or shared users. This field is available in API version 26.0 and later.
Visibility can have the following values.
AllUsers—The file is available to all users who have permission to see the file.
InternalUsers—The file is available only to internal users who have permission to see the file.
SharedUsers—The file is available to all users who can see the feed to which the file is posted. SharedUsers is used only for files shared with users, and is available only when an org has private org-wide sharing on by default. The SharedUsers value is available in API version 32.0 and later.
For more information, refer ContentDocumentLink
Update based on comments
List<ContentDocumentLink> lstlink = [SELECT Id, ShareType, Visibility, LinkedEntityId FROM ContentDocumentLink
WHERE ContentDocumentId = '069q00000001jv0'];
Id objId = cl.LinkedEntityId;
System.debug(objId + 'Object Name=' + objId.getSObjectType().getDescribe().getName());
In the debug log, I get userids.