Add the ability to limit file extensions for ImageField and FileField

[Tim Graham]

ImageField/FileField could have an optional extension whitelist. This could default to being empty for both fields to be totally backwards compatible, or could have sensible defaults for the ImageField based on the file types supported by PIL/low. The documentation should warn that you should not rely on file extensions to determine the actual content type of files.

There are probably some existing implementations we could borrow from, for example:

​https://djangosnippets.org/snippets/977/

[Jason Filipe]

I have a work in progress PR here: ​https://github.com/jfilipe/django/pull/2

Wanted to get some feedback on the approach before I added some docs.

[Tim Graham]

Hi, I left a comment for improvement on the PR, although it's better to open a PR against Django itself instead of your fork.

[ANUBHAV JOSHI]

The patch looks good but the validator must be like others as Tim suggested on your PR.
Also adding it to ImageField by default sounds good. I hope we can fix #18543 with it.

[Berker Peksag]

Pull request: ​https://github.com/django/django/pull/6343

Changes:

• Added two validators: FileExtensionValidator and validate_image_file (this one uses FileExtensionValidator with default values from PIL.Image.EXTENSION).
• Added validate_image_file to ImageField as a default validator.
• Added tests for FileExtensionValidator, model and form validations.
• Added documentation and release notes. I probably need to add some note for ImageField docs too.

[Tim Graham]

Left some comments for improvement.

[Tim Graham <timograham@…>]

In 12b4280:

Fixed #21548 -- Added FileExtensionValidator and validate_image_file_extension.

[Baptiste Mispelon <bmispelon@…>]

In a9215b7c:

Refs #21548 -- Skipped tests that rely on pillow when it's not installed