Welcome to the forum.
Container fields can only store one file at a time, saved in the container itself or as a reference. If you require multiple files for a particular record you will need to setup a new related table that will hold the files for you, the relationship would be based on the UniqueID in your main table. and then you can display the related files using a Portal on your layout, and this way you can have as many files attached to a record as you require.
With regards to displaying a thumbnail instead of an icon, you can do this by inserting as a picture instead of a file, however it will not really be a thumbnail, just a scaled down full sized image, so the database will load the whole image each time.
There are plugins out there like TroiFile that can manipulate pictures and create thumbnails, but these may be a bit too much for what you need, so see how this approach goes.
If you need any clarification on anything then just ask.
thanks so much!