dom-pixels and node-pixels don't accept same type string sets

[benwiley4000]

dom-pixels.js accepts the following types:

• '.GIF'
• anything else

node-pixels.js accepts the following types:

• 'image/png'
• 'image/jpg'
• 'image/jpeg'
• 'image/gif'
• 'image/bmp'

Clearly this is a problem if I want to run the same script on the server and client.

The node-pixels version seems more thought-out - perhaps this should be used instead for the client and the '.GIF' option should be deprecated and phased out in the next major version?