Add @AcceptableExtension validation for file uploads

[lifefire1]

Description

Hi Spring team!

I'm a frequent Spring user and I really enjoy working with it. However, I often find myself writing boilerplate code to validate file extensions when handling file uploads.

To solve this, I've created an @AcceptableExtension annotation that allows easy declarative validation of file extensions.

Example Usage

Before:

@PostMapping("/upload")
public String upload(@RequestParam("file") MultipartFile file) {
    String filename = file.getOriginalFilename();
    String extension = filename.substring(filename.lastIndexOf(".") + 1);
    if (!List.of("jpg", "png", "pdf").contains(extension.toLowerCase())) {
        throw new IllegalArgumentException("Invalid file extension");
    }
    // process file...
    return "success";
}

After:

@PostMapping("/upload")
public String upload(
        @AcceptableExtension(extensions = {"jpg", "png", "pdf"})
        @RequestParam("file") MultipartFile file) {
    // file is already validated
    return "success";
}

What's Included

• @AcceptableExtension annotation with customizable extensions and error messages
• AcceptableExtensionMethodArgumentResolver for validation logic
• Comprehensive test suite (15+ test cases)
• Case-insensitive extension matching

All tests pass successfully.

Thank you for considering this contribution!