PDF Preview

### Your use case

#### What would you like to do?
Most chat apps have a way of previewing basic files directly in the chat client such as PDFs. Element can already do this great with images and videos. But PDFs still have to be first downloaded and then opened with an external editor.


#### Why would you like to do it?
Its much more convenient to get a quick PDF preview in the client than having to go through the extra steps of downloading the PDF and opening it.

#### How would you like to achieve it?
In VsCode etc this feature is already implemented (perhaps as Extensions) but it means there are libraries that can render a the PDF.

Perhaps rendering a preview during the send (as an image) and having it as a little icon next to the PDF?

### Have you considered any alternatives?

At the moment, you can just download the PDF. This is fine, but would be more convenient if one could directly preview the image in the chat....

### Additional context

_No response_

The issue names no files or tests. Start by tracing Element's existing image and video preview flow and evaluating how PDF rendering could fit that client-side path. Done means users can preview PDFs directly in chat without downloading them first.

Written by the indexing model from the issue text.