in the address bar, replace "filename.php" with "HTMLEditor/custom/test_text1.js"
in other words if you browse the page with the HTMLEditor instance to it at:
change it to:
This should open the test_text1.js file in the browser.
If not, please check your site settings.
On the Local info tab, Is the Local Root Folder the same as the Testing Server Folder on the Testing Server Tab?
If they are different, In the Dreamweaver Files Window, select the HTMLEditor folder and click the blue up arrow to upload it to the testing server folder.