Spelling: "MIME-type" and various fixes

[comradekingu]

No description provided.

[ximion]

Hey :)
Sorry for getting back to you that late! I was very distracted in December / end of November, and wanted to give this PR a bit more attention, since it not only resolves typos and the like, but also "style" (e.g. Command '%s' is unknown. --> Unknown command '%s'.).
I originally wanted to make a release in December, and breaking a lot of translations would have been counter-productive at that point.

In any case, thank you for your contribution! The changes look good to me (except for maybe the "item value" one, which I might tweak a little).