External images should not use [[File:...]] syntax.

Inline external image:
```
% pandoc -f native -t mediawiki
[Para [Image ("",[],[]) [] ("https://example.org/image.png", "")]]
^D
<nowiki></nowiki>https://example.org/image.png<nowiki></nowiki>
```

Inline local image (should still use [[File:]]):
```
% pandoc -f native -t mediawiki
[Para [Image ("",[],[]) [] ("local.png", "")]]
^D
[[File:local.png]]
```

Figure with external image (from markdown):
```
% pandoc -f markdown -t mediawiki
![](https://example.org/figure.png)
^D
<nowiki></nowiki>https://example.org/figure.png<nowiki></nowiki>
```

Local image (from markdown, no caption):
```
% pandoc -f markdown -t mediawiki
![](local.png)
^D
[[File:local.png]]
```

Figure with caption and external image:
```
% pandoc -f markdown -t mediawiki
![Caption](https://example.org/figure.png)
^D
<div class="figure">
<nowiki></nowiki>https://example.org/figure.png<nowiki></nowiki>
</div>
```

Figure with caption and local image:
```
% pandoc -f markdown -t mediawiki
![Caption](local.png)
^D
<div class="figure">
[[File:local.png|Caption]]
</div>
```

SimpleFigure with external image (native format with fig: title):
```
% pandoc -f native -t mediawiki
[Figure ("",[],[]) (Caption Nothing []) [Para [Image ("",[],[]) [Str "caption"] ("https://example.org/figure.png", "fig:")]]]
^D
<div class="figure">
<nowiki></nowiki>https://example.org/figure.png<nowiki></nowiki>
</div>
```

SimpleFigure with local image (native format with fig: title):
```
% pandoc -f native -t mediawiki
[Figure ("",[],[]) (Caption Nothing []) [Para [Image ("",[],[]) [Str "caption"] ("local.png", "fig:")]]]
^D
<div class="figure">
[[File:local.png|thumb|none|alt=caption|caption]]
</div>
```