What was the last film you saw [as of your last birthday]? Not technically correct because the second past tense verb should be past perfect, had seen, but you would hear this spoken in the US and people would know what you meant. This is asking what was the last film that you had seen at a previous point in history, before yesterday.
What is the last film you saw? Can you tell me what is the last film you have seen? This is how we would normally ask this question, in conversation, about the present. Technically the second sentence is more correct, because you are asking about an experience, and this continues to be your experience (the last movie that you saw) into the present.
Can you tell me what was the last film that you had seen [as of your last birthday]?
-This would be asking about a point in history that has finished. So maybe you were talking about a day last year, and you wanted to know what was the last film that she saw at that point, a much less likely scenario. You can't use this to ask about her present experience.
So you use "what is" to ask about the present state of experience, but you use "have seen" because the experience has happened before the present moment. The last movie that you have seen happened in the past but continues to be your current experience into the present.
The present perfect tense refers to an action or state that either occurred at an indefinite time in the past (e.g., we have talked before) or began in the past and continued to the present time (e.g., he has grown impatient over the last hour). Formed by have + past participle: have seen. The last movie that you viewed happened in the past but it continues to be the last movie that you viewed into the present.
The PAST PERFECT TENSE indicates that an action was completed (finished or "perfected") at some point in the past before something else happened. Formed by had + past participle: had seen.
I hope this helps.