The way you try to create the sorted keys is incorrect. Try this:
NSString *filmTypePath = [[NSBundle mainBundle] pathForResource:@"photographic-film" ofType:@"plist"];
filmTypesDict = [[NSDictionary alloc] initWithContentsOfFile:filmTypePath];
filmTypesArray = [[filmTypesDict allKeys] sortedArrayUsingSelector:@selector(compare:)];
And there is no need for any of these arrays to be mutable.
The method you were using is trying to sort the keys based on their corresponding values. You simply want an alphabetic sort of the keys with no regard to their values.