Smart Previewer od Soushi Atsumi

This extension shows contents of links. Audio, images, pdf and videos..., all of links which this extension supports need not to be clicked. You can check it by just putting on the link.

Following types are supported for audio.
"aac", "flac", "m4a", "mp3", "ogg" and "wav".
Following types are supported for images.
"bmp", "gif", "gifv", "ico", "jpeg", "jpg", "png", "svg", and "webp".
Following types are supported for pdf.
"pdf".
Following types are supported for videos
"mov", "mp4", "webm" and "wmv".
These types will be extended in the future.

In version 2.0, this extension supports dynamically generated pages!
All links can be previewed.

You can watch the video of this extension.
Smart Previewer

Why does this extension require "Access your data for all websites"?
This extension is working to override websites displayed by the browser shows. Because of that, the extension needs to read the content of websites. The content read by the extension will never send outside. To eliminate any worries, the source codes are open to the public at the following link, and you can check that it actually does.
https://github.com/Soushi-Atsumi/SmartPreviewer_Firefox

This extension is also available on Microsoft Edge.