Thumbnailed files have little contrast with background

## What Happened


Thumbnailed documents have little contrast with the background, making it very hard to discern where the thumnail starts or ends, in turn making it difficult to click on it. In some cases it is hard to see whether the thumbnail is even there.

## Expected Behavior

https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2Fsupport.apple.com%2Flibrary%2Fcontent%2Fdam%2Fedam%2Fapplecare%2Fimages%2Fen_US%2Fsocial%2Fmacos-mojave-macbook-finder-social-card.jpg&f=1&nofb=1

Add some sort of border and drop shadow to each thumbnail to make them have more contrast with background

## Steps to Reproduce

1. Create a thumbnailable file, like a PDF
2. Observe the thumbnail in Files
3.

### Logs

## Platform Information

elementary OS 6 Odin stable, Files 6.0.1