Retrieve a media using either fileName or media ID

[eben-flow]

I want to be able to retrieve a media using either its file name (in combination with the collection name) or the primary key in the media table.

Here is my use. I have a products model and a product can have multiple images. However, I want one image to be the default image. I'm currently saving the file name of the default image in the product table. Now I want to retrieve the URL or any other information specifically for the default image.