Add support for allowed file extensions

[amirhusein1996]

Add support for allowed file extensions in FileField

In the init method:

• Added handling for 'allowed_extensions' parameter.
• Ensured 'allowed_extensions' is either None or a valid list/tuple.
• If 'allowed_extensions' is provided, added a FileExtensionValidator to the list of validators.

[cclauss]

Please rebase to fix the failing pre-commit test.

[tomchristie]

Things I would want to see us encouraging...

• We start declining functionality pull requests that have not had a prior discussion.
• We encourage our users to advocate for behavioural changes from a documentation-first perspective.
• Comparison with Django... if a change can be shown very simply to be non-breaking and it brings us more closely in line with a Django behavior, then it's a maybe. Otherwise we should strongly default to "no". Most proposed changes do not need to be included in core.