I apologize for forgetting to attach the file. It would have been much easier if I had remembered to include it. I realize this may cause extra work for you, I'm sorry for any inconvenience.

I take full responsibility for forgetting the attachment. I understand that this may be inconvenient for you and I apologize. I will make sure to double check my emails in the future so that this mistake won't happen again.

I apologize for not attaching the file. I should have double checked my email before sending it. I understand that this may have caused you extra work or delays. I will take extra precautions in the future to make sure this doesn't happen again.

I apologize for not including the attachment. I should have been more careful and double checked my email before sending it. I understand this may have caused you extra work, and I apologize for any inconvenience this may have caused.

I am sorry for forgetting to attach the file. I should have been more vigilant in double checking my emails before sending them. I understand that this may have caused you extra work and delays, and I apologize for any inconvenience.