What is a good type of decoupling capacitor (i. e. the ones connecting PSUs and ground and installed close to fast opamps). Ceramic? film? ...

I suppose 0.1uF should be ok in most cases.

 

I have been told ceramic film is good. And indeed 0.1uF or 10nF should be fine. One similar to this from Rapid: Ceramic Radial Multilayer should be fine.

 

Ceramic is very good at high freq, so with their durability and low cost they are a good first choice. Tantalum for bulk bypass are also excellent though more $$$. Tant are better then electrolytic as those don't have a good hi freq response.

I dunno about film caps as I rarely use them.

I keep piles of 0.1 ceramic caps in my box so I am free to sprinkle them all over my circuits.

 

Spec sheets are your friend here and many other cases. Learn to read them because they will provide you with much knowledge of electronic components. A competent electronics person knows the characteristics of components so he may select those with appropriate operating parameters for the circuit in question without over specifying.

Remember Spec sheets are your friend!