What's missing in the thumbnail_fixes branch is the peace of code I qouted earlier. 
```
end($possibeThumnails);
if (key($possibleThumbnails) < $maxX) {
    return current($possibleThumbnails);
}
```
This checks, if the biggest preview available, that has exactly the requested ratio, is smaller than the requested size (, but still with an acceptable scaling up rate).

Because this is missing in thumbnail_fixes and kinda broken in master, scalingup is not working.

> @georgehrke @DeepDiver1975 what do you think?

This protection against malicious users is already given by preview_max_x and preview_max_y