I'd like very much to see a widescreen view mode - where the preview and the file listing are side-by-side. While I'm waiting for that to (hopefully) happen, here's a workaround - best on a big monitor; not so great on a laptop screen.
First, set your EagleFiler window to not show a preview.
Second, size your EagleFiler window to whatever works best for seeing your file listings - I've got it filling the whole screen top-to-bottom, and about 2/3 of the screen side-to-siide.
Next, click on the record you'd like to see. Hit space bar. A preview window will open.
Move the preview window to the right side of the screen and size it to fill up the space the EagleFiler window doesn't fill.
The preview window will stay open, so you've got a side-by-side view.
A kludge, but - for me - a needed one...