Jaycel and I just went through this successfully and she arrived here on her K-1 visa just a couple weeks ago. I took the philosophy that I have always taken with the US Government when asking for them to approve something, which is give them so much evidence that they almost "have to" approve you. I can't speak to the embassy requirements in Frankfurt but as far as USCIS, our I-129F package was 109 pages altogether. We sent about 3 dozen pictures (inserted into a word document with captions), most focusing on time we spent together with her family, pictures of our engagement, pictures of us on our first vacation together, etc.. I sent copies of the entry and exit stamps I received when visiting Jaycel in the Philippines. We also included scanned copies of the boarding passes for my flights to and from the Philippines and also for the flights we took together while I was there, hotel reservations and receipts with our names on it, receipts for gifts I gave her, receipts for the engagement ring, and we did include a sample of our messenger chats over the course of our time together. You will see from my timeline that we did get an RFE but that was stupidity on my part as I had a run-in with the law when I was young and got a conviction. I thought I just had to provide the details, but if you have something like that in your past you have to provide certified copies of the court records, which I did not know. However, since our application was so complete, once they received our RFE response giving them my official court disposition, we got our approval a week later. The embassy gets a copy of your I-129F and we had so much info included that Jaycel's interview lasted about 4-5 minutes total and her visa was issued within 3 days of the interview. Give USCIS enough good proof that they feel no hesitation in approving the application.