UPDATE: I have found a solution. This doesn't necessarily address every case, so I will leave the question open for a short time in case someone can enlighten me more. I solved it by changing the format of the url: Google Drive allows this format for downloading files:
https://docs.google.com/uc?export=download&id=FILE_ID
So I don't know if this is a problem for other URL's; nor actually exactly why the .getDownloadUrl() doesn't work ... maybe someone can explain. But for now this seems to work in the browsers that I can test ...
I have a simple WebApp script which I run on a Google Site by adding the Apps Script gadget. The gadget runs exactly as the Forms example on:
https://developers.google.com/apps-script/guides/html/communication#forms
The gadget is designed to do the following: when the page is loaded, a form is returned, and the user must enter a license key to get a link to download a product. My code serves the form OK, and gets the form submit OK; and it then validates the key, and if valid, sends back a link to download. All that works fine; and the problem is that no matter what I try to return for the download link, the caja iframe wrapper is preventing the click on the link from actually downloading the file.
My preferred URL to return is in fact via the Drive API: the download file is on the Google Drive, and I get the download link like so:
DriveApp.getFileById(downloadFileId).getDownloadUrl()
But when the returned link is clicked inside that caja iframe generated for the WebApp gadget, nothing happens. I have tried a few other URL formats pointing to that file on the Drive, but nothing is working for a download.
Is this possible?
.getDownloadUrl() method returns a temporary URL that can be used to download the file. This URL is valid only for a short period of time, after which it expires and does not return the file any more - that is probably why the links in your web app do not work. Can't remember exactly how long the URL is valid for, but I think it could be as short as 5 minutes.
Permanent download URL is stored in another file property: webContentLink. However, this property is not (yet) available through Google Apps Drive Service - you must use Advanced Drive Service to access it. You can enable Drive API under Advanced Google services in your script. After it is enabled, you can use it like so:
var file = Drive.Files.get(FILE_ID_HERE);
var dlUrl = file.webContentLink;
This will return the link just like the one you found and posted in your update. An advantage of using the Drive API to get the link, instead of hard-coding it, is that if Google ever changes the format of that URL, your code using Drive API to get the link will continue to work, while hard-coded links will not.
Full Drive Web API reference (what Advanced Drive Service uses) is at https://developers.google.com/drive/v2/reference/.