How to select all similar files in an directory If I want to view images I made with my camera using Preview, I have to select all files separately. Because when I select all files (cmd + a) all the videos I made get selected (and opened in Quicktime) too.

So I was wondering how I can select only the images in a certain directory?
You can use the Search feature in Finder to limit the view in a specific folder to search criteria that matches what's in the search field. One of the criteria you can search on is kind. In the screen shot below I start typing kind:jpeg in the search field and you see that Finder presents me with a drop down list that includes JPEG Image. I select this from the list and I'll see only files of that kind. To limit the filter to just this folder I make sure the Downloads view is clicked on the Search: area just about the list of files instead of This Mac.

Once you've searched and filtered your view this way, you can select all to perform an operation on files that match that search.