Limit accept attachment file types, fix missing id and make Trix compatible with modals

[davilajose23]

Context

While integrating Trix, I found a conflict if I wanted to attach files and Trix was inside a modal, as discussed in issue #1039. Additionally, I needed to restrict the attached files to only images. During debugging, I also noticed that the input element was missing an ID, a problem previously reported in issue #1088 by @chadrschroeder. To address this, I monkey-patched it in my implementation and decided to create this pull request.

Changes

1. Add an option to limit the file types when attaching a file.
2. Inserting the input file at the same level as the trix-editor instead of doing it at the end of the body
3. Add the missing fileInputId based on Restore Missing fileInputId Property and Add acceptedFileTypes Property #1089

To solve 2 and 3 I had to use the editorController instance as an argument of the pickFiles function instead of just using the callback.
Let me know if you consider that not all the changes are needed or if there are any issues with the code.

How it works

Trix will still accept all file types as an attachment by default, but now a trix-attachment-accept attribute is available, which takes a comma-separated list of allowed file extensions or MIME types.

<trix-editor trix-attachment-accept="image/png, image/jpg, image/jpeg"></trix-editor>

<trix-editor trix-attachment-accept=".png, .jpg, .jpeg"></trix-editor>

<trix-editor trix-attachment-accept="image/*"></trix-editor>