Clicking to Attach a File Deletes First Element on Page with ID of "undefined"

While overriding the `pickFiles` function, I noticed that there is no `fileInputId` property to provide a unique ID for the dynamically inserted file input element.

Back in Trix version 1, that existed [here](https://github.com/basecamp/trix/blob/1.3.1/src/trix/config/input.coffee#L21) and looked like:

```coffeescript
fileInputId: "trix-file-input-#{Date.now().toString(16)}"
```

The entire `pickFiles` function was removed [here](https://github.com/basecamp/trix/commit/40a8e33f6343971f1ef6d7c0b439925353c32424#diff-b6ecefb73ea0d1f0413e5c9ea055b434f75e7691e984abc475bbf79c9eb2e900). Then that function was added back without `fileInputId` [here](https://github.com/basecamp/trix/commit/2c6c48080c4844b9ba0b2feda171215c080ecb27#diff-2307591fa6d5a739a31c8198158d9d5f30fdec71af33029947e927bda0bff640).

This means that if for some strange reason you had another element on the page with an `id` of "undefined", clicking to add a file in Trix would remove that element.

Also, as a side question--would you be open to a pull request that adds a way to configure the `accept` property on the `pickFiles` input element? I need to restrict the types of files a user can select. I know I can check the file type in the `trix-file-accept` event and call `event.preventDefault()` there, but that happens after the user has already selected a file. It would be nice to be able to restrict the list of possible files while they are making the selection. (**Update:** I now see pull request https://github.com/basecamp/trix/pull/789 addresses this and has been open for over 3 years.)

##### Details

* Trix version: 2.0.5

Start by locating the current pickFiles entry point and compare it with the linked Trix 1 input configuration. Reproduce the case with another page element whose id is "undefined", then verify that choosing a file no longer removes it. The separate accept-attribute request is already addressed by the linked pull request and should be treated independently.

Written by the indexing model from the issue text.