Add support for webp images to insert image flow

[jpd236]

Uploading .webp images via the "Insert image" flow seems to fail with an error in Document.methods.insertImage:

FetchError · invalid json response body at https://script.google.com/[snip] reason: Unexpected token < in JSON at position 0

A couple of users hit this with our test run via Galactic Puzzle Hunt. If it's not too much trouble, it seems like it would be nice to support this as it's becoming more prevalent.

[jpd236]

The app script is failing on line 132 (the call to sheet.insertImage) with the error "Error retrieving image from URL or bad URL". For whatever reason, despite the script failing, it returns an HTTP 200 response with an HTML-formatted error. So the failure mode is in trying to parse the response as JSON because it is supposedly "ok".

It's not explicitly stated, but I'm guessing that only PNG/GIF/JPG images are supported. For the short term, we can probably just restrict the formats on the upload/link side as well so the failure is clearer. Longer term, I suppose JR could convert images to a supported format before uploading.

Note that copy-pasting the image works just fine.