fix(preview): update mime-type check for json files (#2221)

[KevinSilvester]

Description

Fix JSON files without file extensions being treated as non-text files as its MIME type is application\json.
The current implementation splits the output of the file command and only uses the first item of the split output to check if file reviewable.

Also manually setting opt.ft to json for syntax highlighting as plenary.filetype.detect is unable to recognize JSON files without file extension

Issue #2221

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Create a JSON file with no file extension and some dummy data

{
   "dummy": "data"
}

• Telescope find_files and verify the file is previewable and syntax highlight is working

Configuration:

• Neovim version (nvim --version): NVIM v0.9.0
• Operating system and version: Windows 11 version 22H2 (Build No: 22621, Arch: x86_64)

Checklist:

• My code follows the style guidelines of this project (stylua)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (lua annotations)

[Conni2461]

thanks :)