routing doc - handling file uploads

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i just wanted to try doing a quick file upload... but found the topic, all in all, somewhat trickier than what i would have hoped it'd be... so.... i guess it would be really great if the doc on this could be a little bit more... well... elaborate....

in any case.... one thing that i found particularly confusing is the following:
```
(fn [{{{:keys [file]} :multipart} :parameters}]
```
i personally find this sort of destructuring-syntax super hard to parse.... also.... in my case the path turned out to be more like :params -> :file rather than :parameters -> :multipart -> :file

also... how does all of this fit in with what it says just a few lines above:

```
Reitit supports three kinds of parameters. These can be route, query, and body parameters.
```

but like i said.... for me as a total file-upload-noob :-).... all of this is still pretty confusing... :-)

Start with the routing documentation section containing the file-upload example and its parameter overview. Clarify the destructuring syntax, explain the expected parameter path for uploads, and show how file uploads relate to route, query, and body parameters; done means a newcomer can follow the example without the reported ambiguity.

Written by the indexing model from the issue text.