How to Make Files on Records Visible to Customers by Default / Automatically

Apex Trigger:

trigger ContentDocumentLinkTrigger on ContentDocumentLink (after delete, after insert, after undelete, after update, before delete, before insert, before update) {
    ContentDocumentLinkHandler.ProcessContentDocumentLinks();
}

Apex Handler Class:

global with sharing class ContentDocumentLinkHandler {
    // Debug walkthrough indicator
    static String showDebugs = 'walkthrough';  
    // Change value to 'walkthrough' to view high-level debugs steps in Log, 
    // 'fine' to include all debugs, and 'None' to exclude debugs
    global static void ProcessContentDocumentLinks() {
        // All trigger requests enter here.  Provide routing to the correct
        // method for processing.  
        IF (!Trigger.isExecuting) {
            system.debug('ContentDocumentLinkHandler > ProcessContentDocumentLinks > '
                + ' Error: The ProcessContentDocumentLink() Trigger Handler method was' 
                + ' just called with no active ContentDocumentLink Trigger in process. ' 
                + ' The ProcessContentDocumentLink() method may ONLY be called during a' 
                + ' Trigger Context.  Other methods can be called '
                + ' at any time, but not this Trigger Traffic Cop method.');
        }
        else {
            if(showDebugs != 'none') system.debug('ContentDocumentLinkHandler > ProcessContentDocumentLinks > Trigger Size is: ' + Trigger.size);
            if(showDebugs == 'fine') system.debug('ContentDocumentLinkHandler > ProcessContentDocumentLinks > Trigger.new: '+Trigger.new);
            if(Trigger.isBefore){
                if(Trigger.isInsert){
                    BeforeInsert(Trigger.new, Trigger.newMap);
                }
            }
            // Unused conditional criteria commented-out, but left
            // here for ease of use in case of future development
            /*
            if(Trigger.isAfter){
                if(Trigger.isInsert){
                    AfterInsert(Trigger.old, Trigger.new, Trigger.oldMap, Trigger.newMap);
                }
                if(Trigger.isUpdate){
                    AfterUpdate(Trigger.old, Trigger.new, Trigger.oldMap, Trigger.newMap);
                }
                if(Trigger.isDelete){
                    AfterDelete(Trigger.old, Trigger.new, Trigger.oldMap, Trigger.newMap);
                }
                if(Trigger.isUndelete){
                    AfterUndelete(Trigger.old, Trigger.new, Trigger.oldMap, Trigger.newMap);
                }
            }
            */
            }
        }
        return; 
    }
    public static void BeforeInsert(List<sObject> newContentDocumentLinks, Map<Id, sObject> newContentDocumentLinkMap){
        ContentDocumentLinkUtil.BeforeInsertActions(newContentDocumentLinks);
    }
}

Apex Utility Class

public without sharing class ContentDocumentLinkUtil {
    // Debug walkthrough indicator
    static String showDebugs = 'Fine';  
    // Change value to 'walkthrough' to view high-level debugs steps in Log, 
    // 'fine' to include all debugs, and 'None' to exclude debugs
    public static void BeforeInsertActions(List<SObject> newContentDocumentLinks){
        if(showDebugs != 'None') system.debug('ContentDocumentLinkUtil > ' +
                                              'beginning convertShareTypeToViewer');
        // Convert all ContentDocumentLink Visibility to 'AllUsers' to allow
        // Customer Community User access by default.
        for(Integer i = 0, j = newContentDocumentLinks.size(); i < j; i++){
            newContentDocumentLinks[i].put('Visibility','AllUsers');
        }
    }
}

Note that in the Apex Utility Class, you could apply conditional logic to only have the Customer Community access be on certain objects. As an example:

for(Integer i = 0, j = newContentDocumentLinks.size(); i < j; i++){
    if(newContentDocumentLinks[i].LinkedEntityId.getSObjectType().getDescribe().getName() == 'Case'){
        newContentDocumentLinks[i].put('Visibility','AllUsers');
    }
}