Custom thumbnail images with a fallback to default ones

Hi all, I want to set up my SharePoint view (first the List, later also the Tiles view) so that it would show thumbnails for each file and folder.

Some files in the folders already show a default thumbnail (e.g. images and some 3D formats) but for some, I have uploaded an image. I'm struggling to set up the SharePoint view so that it would show this custom image, and if it is missing, fall back to a default thumbnail (if available). 

By applying the following code snippets to a "Thumbnail" column I can make either the thumbnails show or the custom images, but not together in wished the logic. Could there be a bug? Or does anyone know a workaround for this? 

Thanks in advance for any help and hints. 

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Here are my trials:

Test A) WORKS AS EXPECTED - the custom image is shown
(Note: only one out of the 6 files in the folder have a custom image uploaded):

{
  "$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
  "elmType": "img",
  "attributes": {
    "src" : "=getThumbnailImage([$Thumbnail], 100, 100)" 
    // alternatively: 
    //"src" : "@currentField.serverRelativeUrl"
  }
}

Test B) WORKS AS EXPECTED - default thumbnails are shown

(Note: all 6 files have default thumbnails/previews):

{
  "$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
  "elmType": "img",
  "attributes": {
    "src" : "@thumbnail.small”
  }
}

Test C) WORKS AS EXPECTED - print a div with either "uploaded" or "default" text

{
  "$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
  "elmType": "div",
  "txtContent" : "=if(@currentField, 'uploaded', 'default')"
}

Test D) DOESN'T WORK - show either uploaded or default thumbnail

{
  "$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
  "elmType": "img",
  "attributes": {
    "src" : "=if(@currentField, @currentField.serverRelativeUrl, @thumbnail.small)"
  }
}

Test E) DOESN'T WORK - print a div with text field. If the IF-statement includes anything currentField related (e.g. serverRelativeUrl), the previously working conditional doesn’t catch the “default” case anymore:

{
  "$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
  "elmType": "div",
  "txtContent" : "=if(@currentField, @currentField.serverRelativeUrl, 'default')"
}