@Alpha2020 Here you go
For Get List :
import com.google.appinventor.components.runtime.util.YailList;
import java.util.ArrayList;
import java.util.List;
................
.........
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@SimpleFunction(description = "")
public YailList GetList() {
List listItems = new ArrayList();
listItems.add("Text1");
listItems.add("Text2");
listItems.add("Text3");
return YailList.makeList(listItems);
}
Thank you very very much..... 
Note that you can also return type List<> directly, for example:
public List<String> GetItems() {
return Arrays.asList("Item1", "Item2", "Item3");
}
The App Inventor runtime will take care to ensure that every value returned in the list is coercible to a valid type in the blocks editor.
If you return YailList, you are expected to ensure that every item in the list is valid.
Sorry to drag this one up from the deep.....
Is it possible to return a list of lists (list of pairs)? For example like this:
return Arrays.asList(("Item1",1), ("Item2",2), ("Item3",3));
My IDE doesn't like the syntax....
maybe? -
return Arrays.asList(Arrays.asList("Item1",1), Arrays.asList("Item2",2), Arrays.asList("Item3",3));
Hope i did not misunderstood your question or gave a incorrect answer 
Well the IDE likes it, will have to build and see what comes back 
and it comes back as a list of lists 
@SimpleProperty(description = "list of mimetypes")
public List Mimetypes() {
return Arrays.asList(Arrays.asList("PNG", "image/png"),Arrays.asList("JPG", "image/jpg"),Arrays.asList("BMP", "image/bmp"));
}
(only problem is, have you seem how long the mimetypes list is 
- not really a problem, I will need to use find and replace anyway)
If you have the mime list as a CSV file you could read it into the extension and pass its contents through CsvUtil.fromCsvTable (which is the what the list from csv table block does).
Like so ?
@SimpleProperty(description = "list of mimetypes")
public List Mimetypes() {
return CSvUtil.fromCsvTable("PNG", "image/png"\n"JPG", "image/jpg"\n"BMP", "image/bmp");
}
