feat: add supported extensions in appsettings

[ElenaStamenkovskaCC]

What Changed?

• This PR adds a fluent validator for FileUploadRequest. It check the validity of the files based on their extensions. It also checks for valid signature of the extension.

Why?

• To improve security by preventing users from uploading files with fake extensions.
• Ensures only supported file types are accepted, reducing potential runtime errors or security risks.

Closes #261

Type of Change

• 🐛 Bug fix (fixes an issue without breaking existing functionality)

How Did You Test This?

• Tested this by uploading files to some test collections, with Swagger.

What I tested:

• [ ]
• [ ]

How to test it:

Review Checklist

• My code follows the project's style and conventions
• I've reviewed my own code for obvious issues
• I've added comments where the code might be confusing
• I've updated relevant documentation (if needed)
• I've tested my changes and they work as expected

Screenshots / Additional Context

[fvanderflier]

https://code-maze.com/aspnetcore-how-to-validate-file-upload-extensions/