setFile "fileInputId", getResourcePath("some photo.jpg")

getResourcePath() – returns path to resource (java/groovy resources meant). You could use any path string instead, but inclusion of resources (test resources) is more convenient, because they become part of your test environment. If you have some problems with java/groovy resources, feel free to ask.

* We will extend our SeleniumTest (basic class for all integration tests with this method:

public abstract class SeleniumTest extends SeleneseTestCase {
    static String context = "http://localhost:8080/app/"

    public void setUp() {
        setUp context, "*firefox"
    }
    ...
    void setFile(String fileInputId, String fileName) {
        selenium.getEval(
            """var imageControl = window.document.getElementById('${fileInputId}');
               imageControl.value='$fileName';
               var event = window.document.createEvent('HTMLEvents');
               event.initEvent('change',true,true);
               imageControl.dispatchEvent(event);
            """)
    }

    String getResourcePath(String resourcePath) {
        getClass().getResource(resourcePath).getPath()
    }
    ...
}

This method uses document model to inject file path into HTML file input control and also generates onChange event, which could be useful if onChange event handler defined for this component.