A simple event extension on the component save initiated, part of blogpost: http://jarnohenneman.com/2014/06/BinaryUpload-event/

BinaryUpload.cs

using System;
using Tridion.ContentManager.ContentManagement;
using Tridion.ContentManager.ContentManagement.Fields;
using Tridion.ContentManager.Extensibility;
using Tridion.ContentManager.Extensibility.Events;
using Tridion.Logging;

namespace Trial.and.Error.BinaryUploadEvent
{
    [TcmExtension("BinaryUploadEvent")]
    public class BinaryUpload : TcmExtension
    {
        public BinaryUpload() 
        {
            Subscribe();
        }

        public void Subscribe() 
        {
            EventSystem.Subscribe<Component, SaveEventArgs>(OnBinaryUpload, EventPhases.Initiated);
        }

        private static void OnBinaryUpload(Component comp, SaveEventArgs args, EventPhases phase)
        {
            //Logger.Write("[Events.ComponentSave]", "ComponentSave has been called for " + comp.Title, LoggingCategory.General);

            String schema = "Multimedia";
            String field = "altText";
  
            if (comp.ComponentType == ComponentType.Multimedia)
            {
                if (comp.Schema.ToString().Equals(schema) && comp.Metadata != null) //[1]
                {
                    ItemFields metaFields = new ItemFields(comp.Metadata, comp.MetadataSchema);
      
                    if (metaFields.Contains(field) && String.IsNullOrEmpty(metaFields[field].ToString())) //[2]
                    {
                        TextField altText = metaFields[field] as TextField;
                        altText.Value = comp.Title;

                        comp.Metadata = metaFields.ToXml();
                    }
                }
            }
        }
    }
}