Does not work on Firefox with NoScript addon - even if github.com is explicitly allowed, and even if NoScript is set to "Allow Scripts Globally (dangerous)", one still gets in Error Console (Ctrl+Shift+J) in Firefox:
[NoScript] Blocking nosniff Javascript served from https://raw.github.com/Manishearth/Manish-Codes/master/StackExchange/ThumbnailUpload/ThumbnailUpload.js?rand=16 with wrong type info text/plain, inline and included by Thumbnail Uploader
Here is the relevant post from NoScript forums:
InformAction Forums • View topic - nosniff is blocking a script without telling me
I can't think of any reason why NoScript should block scripts without indicating that it is doing so. It just leads to frustration.
[...]
Not sure it's worth the effort: careless developers like that will soon be forced adapt, since IE9 doesn't give any visual clue either of this (correct) blocking [...]
Well, that's a shame :/ ...