[UX] Preview area: Thumbnail link to open document / pdf is way too small

[jnachtigall]

There are various issues here, in order of priority:

1. Many users do not find how to open the file because the fallback thumbnail is way to small. I heard this from many people in our group: "I went to the page you sent me, but there was no file"
2. The thumbnail should have a title attribute, so that hovering with the mouse over it says something like "Click to open"
3. Thumbnail generation is frequently failing but this is another issue (Thumbnail generation fails for PDFs or OpenDocument #385).

Thumbnails itself are nice but not that important, having a tiny thumbnail so that people do not find the actual file is a real showstopper.

For context: We frequently upload OpenDocument, PDFs but also gnucash files.