[3.x] Fix binary checks (port of #15900) to make sure textual-ish files can be edited

[Mark-H]

What does it do?

This adds a whitelist of mime types that should be editable. Port of #15900 for 3.x.

Also corrects the logic for calls to isFileBinary; a binary file is one that consists of garbage text and should not be edited, but that logic had been flipped in 3.x.

Why is it needed?

This check is used to decide wether or not the files tree should offer the edit option. Some of these, especially json and xml, are not offered the edit option regardless of exension.

How to test

Add a file with some JSON content, for example:

{
   "style_formats":[
      {
         "title":"Red text",
         "inline":"span",
         "styles":{
            "color":"#ff0000"
         }
      }
   ]
}

Note that in the files tree it is not editable (clicking it does nothing).

Apply the patch, and note that now it can be edited.

Related issue(s)/PR(s)

None, was reported in slack and I went straight from debugging to fixing.