Add file widget as secondary type for the image field inference

**Is your feature request related to a problem? Please describe.**
I have a thumbnail field that can be an image, webm, or mp4 file. Therefore, it is necessary for me to use the file widget with the name and label "thumbnail". The grid view in Netlify CMS uses field inferences to display a preview image for each item in a collection. However, the field inferences code only looks for values from the image widget, not similarly named file widgets.

Relevant code is here:
https://github.com/netlify/netlify-cms/blob/460d1e6835af122f86278b4b2f9c2e3b7bc91488/packages/netlify-cms-core/src/constants/fieldInference.tsx

**Describe the solution you'd like**
Perhaps the file widget can be added to 'secondaryTypes' for the image field inference. If the file widget value is a valid image path then the image should be displayed in the grid view for collections.