add an optional category to be inserted into the path

[rngadam]

This lets us group attachments (for a particular customer or event).

Given this configuration:

ImageSchema.plugin(attachments, {
    directory: "/home/rngadam/letsface/src/prototype/public/images",
    storage : {
        providerName: 'localfs',
        options : {
          removePrefix: "/home/rngadam/letsface/src/prototype/public"  
        }        
    },
    properties: {
        image: {
            styles: {
                original: {
                    // keep the original file
                },
                thumb: {
                    thumbnail: '100x100^',
                    gravity: 'center',
                    extent: '100x100',
                    '$format': 'jpg'
                },
                detail: {
                    resize: '400x400>',
                    '$format': 'jpg'
                }
            }
        }
    }
});

output of image property is:

 image: 
   { detail: 
      { depth: 8,
        format: 'JPEG',
        oname: 'displayed.png',
        mtime: Tue Oct 22 2013 14:27:03 GMT+0800 (CST),
        ctime: Tue Oct 22 2013 14:27:03 GMT+0800 (CST),
        size: 15957,
        path: '/home/rngadam/letsface/src/prototype/public/images/zO0vt4yIo/detail/Sbz4kipeRM-detail.jpg',
        defaultUrl: '/images/zO0vt4yIo/detail/Sbz4kipeRM-detail.jpg',
        dims: [Object] },
     thumb: 
      { depth: 8,
        format: 'JPEG',
        oname: 'displayed.png',
        mtime: Tue Oct 22 2013 14:27:03 GMT+0800 (CST),
        ctime: Tue Oct 22 2013 14:27:03 GMT+0800 (CST),
        size: 2952,
        path: '/home/rngadam/letsface/src/prototype/public/images/zO0vt4yIo/thumb/Sbz4kipeRM-thumb.jpg',
        defaultUrl: '/images/zO0vt4yIo/thumb/Sbz4kipeRM-thumb.jpg',
        dims: [Object] },
     original: 
      { depth: 8,
        format: 'PNG',
        oname: 'displayed.png',
        mtime: Tue Oct 22 2013 14:27:01 GMT+0800 (CST),
        ctime: Tue Oct 22 2013 14:27:01 GMT+0800 (CST),
        size: 77825,
        path: '/home/rngadam/letsface/src/prototype/public/images/zO0vt4yIo/original/Sbz4kipeRM-original.png',
        defaultUrl: '/images/zO0vt4yIo/original/Sbz4kipeRM-original.png',
        dims: [Object] } },

[nuarhu]

Sorry for the late reply.

The change looks clean. Your example, however, does not show the usage of 'category'. I'm still not sure how it adds any value in addition to what is achieved by the keys like 'thumb', 'original', 'detail' etc. Is this really a commonly required use case?