I understand your question ;)
These are the steps you can follow: (It can look like a lot but would work for sure)

1) First create a field named total_attachments__c on an Opportunity Object.
2) Go to Developer Console -> File --> New --> Apex Trigger.
3) Name it CountAttachOnOpp and choose SObject as Attachment
4) Now you will see the trigger got created with few lines of code. Get rid of those few lines of code and Copy and paste the below code.
5)

Trigger CountAttachOnOpp On Attachment (After Insert, After Update, After Delete, After UnDelete){
    
    List<Id> oppIds = New List<Id>();
    
    If(Trigger.IsInsert || Trigger.IsUpdate || Trigger.IsUnDelete){
        
        For(Attachment att : Trigger.New){
            If(att.parentId != null && att.ParentId.getSObjectType() == Opportunity.getSObjectType()){
                oppIds.add(att.parentId);
            }
        }
    }
    If(Trigger.IsDelete){
        For(Attachment att : Trigger.Old){
            If(att.ParentId != null && att.ParentId.getSObjectType() == Opportunity.getSObjectType()){
                oppIds.add(att.ParentId);
            }
        }
    }
    
    List<Opportunity> oppFinalListToUpdate = New List<Opportunity>();
    
    List<Opportunity> FetchingEachOppFromOppIds = [Select Id, total_attachments__c, (Select ParentId FROM Attachments) FROM Opportunity WHERE Id = :oppIds];
    
    For(Opportunity EveryOpp : FetchingEachOppFromOppIds ){
        EveryOpp.total_attachments__c = EveryOpp.Attachments.size();
        oppFinalListToUpdate.add(EveryOpp);
    }
    
    try{
        If(!oppFinalListToUpdate.IsEmpty()){
            update oppFinalListToUpdate;
        }
    }
    Catch(Exception e){
        system.debug('Thrown Error: ' + e.getMessage());
    }
}

6) Now, create a validation rule on opportunity object with below formula:

AND( 

Probability >= 0.75, 
OR( 
ISBLANK(Total_Attachments__c ), 
Total_Attachments__c = 0 
) 

)

You are good to go!!