It drops the second i if a suffix of -V(C) is added.

filim

filmi (accusative of film)

filme (dative of film)

filmin (genitive of film)

etc.

We have similar examples of Arabic borrowings

(Arabic wasf) vasıf but vasfı vasfında etc.

(Arabic kasd) kasıt kasdı (notice the consonant mutation also) etc

(Arabic keyf) keyif keyfim keyfi etc

...

Thank you so much!

linguistics textbooks we have are not always correct! that´s why I needed to double check. 

Thank you so much for the full detailed explanation!

The rule is generally true of Turkish phonology, and often this was resolved by adding a vowel before the troublesome consonant cluster. However as such words have become more common, pronouncing them without an added vowel has become more prestigious. So while a word like "skubria" became "Uskumru" and Sparta and Smirni became "Isparta" and "Izmir," today it would sound very provincial to say "ispor" instead of "sIpor" or "istres" instead of "sİtres".

Another such rule is that real Turkish words never begin with L or R. And in Anatolian poetry and speech in some areas you see "ilmon" instead of "limon" and "ireyhan" instead of "reyhan," but standard Turkish has now left this phenomenon behind.