"Manually create an external image" instructions do not work 5.0.14-prerelease

In revision e3dc00573d9ea4db61e619b8f101168d1b4920a5, the following instructions for manually creating external images in the [ExternalImages](http://tiddlywiki.com/#ExternalImages) tiddler seem to not work.

> To manually create an external image just create the tiddler with the appropriate image content type, and add a _canonical_uri field with a URI pointing to the actual image location.

### Steps to reproduce:

1. Copy the image file into place as "Page Composition.svg".
2. Open up the "TiddlyWiki Architecture.svg" tiddler
3. Create a new Tiddler
4. Manually enter the tags from TiddlyWiki Architecture.svg into the new Tiddler.
5. Manually create a `_canonical_url` field, copy the value from TiddlyWiki Architecture.svg, and edit the value to `./images/Page%2520Composition.svg` (confimed as correct by this snip of Python code in a shell one-liner:

  ```
  % python <<< "import urllib, os; print os.path.isfile(urllib.unquote(urllib.unquote('./images/TiddlyWiki%2520Architecture.svg'))), os.path.isfile(urllib.unquote(urllib.unquote('./images/Page%2520Composition.svg')))"
  True True
  ```
6. Set the type to `image/svg+xml`
7. Name the new tiddler "Page Composition.svg"
8. Click "Save this tiddler"

### Expected result:
The saved tiddler should show the image and, when edited again, should reproduce TiddlyWiki Architecture.svg's "This is an external tiddler..." form of the content box.

### Actual result: 
The tiddler renders as empty and re-editing it reproduces the same "internal text tiddler" content view that it started with.