Adding accept attr for .nupkg upload to guide the user

[robertmuehsig]

I noticed that the manual package upload will accept anything and will fail after the postback if the file extension is not a .nupkg:

To guide the user to their own luck I added the accept attribute, which will work on most browsers:
http://caniuse.com/#search=accept

This is not a security feature - the security part is already handled on the server - it's just cosmetic. The result (in supported browsers) is this:

[yishaigalatzer]

@robertmuehsig thanks for being awesome!

[maartenba]

This is cool! Thanks @robertmuehsig