Open with "default application"... for image/file previews; or better way to handle/scale images.

**Is your suggestion related to a problem? Please describe.**
I am keeping Riot as a smaller window (about 1/4 of the screensize)...therefor all image previews are even more limited in size, because their max. size is a small fraction of the actual application window. This makes it difficult to see details in images. Currently, only "Download as file" is offered, which makes it pretty cumbersome: Preview image - download as file - save it to some location...open File browser, browse to the location, then open file. Takes a lot of time and steps.

**Describe the solution you'd like**
Looking for a more convenient way to open files (not only "download as file"), but for images/files open with "default application" would be helpful. Or Preview in full screen for images maybe?

The issue does not name a file, test, or entry point. Start by locating the image/file preview UI and its existing “Download as file” action, then determine whether opening with the default application or a larger image preview is feasible in this web client. Done means one clearly defined interaction is implemented and its preview behavior is covered by the relevant UI checks.

Written by the indexing model from the issue text.