Mantra currently forces a certain width/height ratio for the header image, which is the total width of the theme VS the header height, which can be set in the theme's Layout settings.
If a user attempts to upload a logo, which is exactly the height of the header but of much smaller width e.g. 300px instead of e.g. 1050px, through the "Header" section of "Appearance" menu Mantra will NOT let him use that image in its full height, but FORCE the aforementioned ratio.
To this effect, he will only be able to chose a small strip of the logo image, instead of displaying it as is.
There is no such option as "don't crop and publish" - as happens with other themes - but cropping that image to that ratio is made compulsory.
So using a logo with Mantra is impossible, unless one is willing to resort to questionable measures, like creating a full width image (e.g. 1050px) that is transparent and placing his logo on top of it.
So the logo will show at its full dimensions, but the header image will be of many more tens of KBs.
There must be a way out of this strange rule, which is so contradicting the otherwise plentiful and generous customization options of Mantra.