Add support for importing .md files

[belcherj]

Fix

Fixes: #2155

Adds support for .md files. Since they are basically plaintext files we can import them using the plaintext editor.

Test

1. Test importing markdown files
2. Test importing plain text files.

[codebykat]

This worked great.

I have two tiny nits:

1. I think we should default the "enable Markdown" checkbox to checked when a .md file is selected:

2. The change in line length makes this line break awkwardly:

Maybe we can remove the word "plain" or make the line longer by saying "plain-text or Markdown-formatted files":

I'm now feeling uncertain if it should be "plain-text" or "plain text" 🤷‍♀️

[belcherj]

I changed the text to read Choose one or more text files (.txt, .md)

[belcherj]

I think we should default the "enable Markdown" checkbox to checked when a .md file is selected

There is a bit of complexity in doing this. I don't think the benefit is worth the cruft it adds. I think we should tackle that when we refactor settings as part of spring cleaning.

[codebykat]

It seems quite silly to not default to setting Markdown on a Markdown file, but it's not a hill I'm choosing to die on today.