Default thumbnails appear stretched for images in landscape orientation

By default the CMS creates a thumbnail for each uploaded image by scaling down the original. The scaling is done such that the width and height are equal to or less than 60 pixels while maintaining the original aspect ratio. However, the HTML adds an attribute "height=60" to each thumbnail shown. As a result, the thumbnails of all images in landscape orientation are scaled up by the browser, which looks ugly.

Steps to reproduce:
1. upload an image of 600x100 pixels
2. locate the image in the image gallery and look how its thumbnail is rendered in the list

Expected behavior: the thumbnail icon looks good (i.e. some 60x60 pixels summary of the original image)
Actual behavior: the thumbnail icon looks bad (the icon is 60x10 pixels, of which the leftmost 10x10 pixels are blown up 6 times)