File upload within the Acumatica system is done through the UploadFileMaintenance graph. The data record that is referenced is UploadFile

You can accomplish your goal of "run a method when a file is attached to a document in Acumatica" a variety of ways.

You can add an event handler to UploadFileMaintenance via an extension as seen below

public class UploadFileMaintenanceExtension : PXGraphExtension<UploadFileMaintenance>
{
    public virtual void __(Events.RowInserting<UploadFile> e)
    {
    }

    public virtual void __(Events.RowInserted<UploadFile> e)
    {
    }
}

Actions can then be determined based on the files origination information ect.

Similarly you can add an event for file saving specific to PO with the following

public class POOrderEntryExtension : PXGraphExtension<POOrderEntry>
{
    public override void Initialize()
    {
        PXGraph.InstanceCreated.AddHandler<UploadFileMaintenance>((graph) =>
        graph.RowInserting.AddHandler<UploadFile>((sender, e) =>
        {
            //Your code here
        }));

        base.Initialize();
    }
}