Adding icon images to the file upload

Hello, I will try to be precise as possible.

i have a simple table
documents:
ID - auto increment

document Name

file
In the screen "file: view a"s I see excellent representation of the file types via icons: for example:
if I upload word document, asprunner will display the word icon, if I upload power-point asprunner displays the ppt icon etc.(and there is also a preview)
where in the application I can ADD custom icons?
I need to add several types
I already have 8 different icons, I assume I have to save them in the images folder of the application,

but I do not know where in the application I can add them so that the application will recognize them as options?