The System.AssertEqual (); is not returning a PASS in my Apex Test Class.
This is because, the field EmailMessage.HasAttachment (standard field) is not being updated to True by Salesforce (this is a protected Read-Only field of Salesforce).

Here is my apex test class:

@isTest static void insertingEmailMssgWithAttachment() {
        Case c = new Case();
        c.Subject = 'Caso de ejemplo';
        c.Status = 'New';
        c.Origin = 'Phone';
        c.Hidden_Has_Attachment__c = FALSE;
        insert c;
        
        EmailMessage e = new EmailMessage();
        e.Subject = 'Email de ejemplo';
        e.ParentId = c.Id;
        insert e;
        
        Attachment a = new Attachment();
        a.Name = 'Adjunto de ejemplo';
        a.ParentId = e.Id;
        Blob b = blob.valueOf('test');
        a.Body = b;
        insert a;
        
        EmailMessage myEmailMssg = [SELECT Id, HasAttachment FROM EmailMessage];
        System.debug('FROM Test Class HasAttachmentCaseEmail_Test - Display myEmailMssg: '+myEmailMssg);
        System.assertEquals(True, myEmailMssg.HasAttachment, 'Error mssg: The outcome should be True but its not');
        
        Case myCase = [SELECT Id, Hidden_Has_Attachment__c FROM Case];
        System.assertEquals(True, myCase.Hidden_Has_Attachment__c, 'Error mssg: The HiddenHasAttachment field should be True but its not');
    }

The following apex trigger contains an if condition:
if(em.HasAttachment == TRUE) {...code...}

and unfortunately, when running the test class, it is not running the code depending on that if condition.
 

Set<Id> emailIds        = new Set<Id>();
.....

        //==> Verify if EmailMessage has a child Attachment & if its parentId is of Type Case
        for (EmailMessage em : trigger.new){
            System.debug('FROM: EmailMessageTrigger - 2ondtrigerNew: '+trigger.new);
            if(em.ParentId.getSobjectType() == Case.SobjectType) {
                System.debug('FROM: EmailMessageTrigger - ParentId is SobjectType = True?-->Yes!');
                if(em.HasAttachment == TRUE){
                    System.debug('FROM: EmailMessageTrigger - Email HasAttachment field = TRUE? --> Yes!');
                    emailIds.add(em.Id); 
                }
            }
        }
          System.debug('FROM: EmailMessageTrigger - set of all inserted/updated emails: '+emailIds);

When doing manual tests of this functionality (adding an Attachment to the EmailMessage attached to a Case), such manual test works correctly (it passes OK).

Can you possibly help?

Thank you very much.