13 - File API Browsers - Add support for display of image thumbnails/previews (pre-upload)

[rnicholus]

This includes:

• 1 or 2 UI choices for displaying previews for images client-side. Possibly the existing vertical file list w/ previews as well as a gallery view.
• Need default thumbnails for non-images and images that cannot be previewed
• Will need to include FUB mode support to make displaying image previews in a completely custom UI simple as well. FU mode will likely dog food this infrastructure.
• option to map MIME types to default thumbnails, support for wildcards

[rnicholus]

Depends on #867

[rnicholus]

The primary goals for this feature/story are:

• Make it simple for core and UI integrators to provide client-side image previews for all modern (File API supporting) browsers.
• UI-only: Display previews, by default, in the default template.
• UI-only: Display a placeholder image while preview is being generated, and a different placeholder if a preview cannot be displayed.
• UI-only: Allow integrators to provide their own "no preview" placeholder images by MIME type.
• (secondary): Provide an alternate "gallery" template for larger more-image centric apps
• (future?): Allow integrators to specify max dimensions for previews?

To implement:

• Draw (depending on the template properties) a <canvas> or <img> element with the appropriate placeholder when the file is selected.
• Update the element with the preview or the "can't display" placeholder after the preview generation attempt has completed.
• Provide a preview option that includes a placeholder property (object). This can contain a map of placeholder image paths by MIME type. Wildcards will be permitted.
• Allow for the server to return a URL for a preview image in the upload response (for previews that cannot be generated client-side, or if the server wants to generate an alternate or higher-quality preview server-side).
• Provide a drawThumbnail method that draws the preview onto either a provided <canvas> or an <img>. This will have a promissory return value with success and failure fulfillment depending on the outcome of the preview generation.

[rnicholus]

Note that this feature includes a change that ensures a "qqfilename" parameter is always sent with all traditional endpoint upload requests. This was done to make it easier and more consistent for servers to parse the file name without having to strip off path information that IE may send along with the name associated with the file's multipart boundary filename field.