Server-Side Validation

Combine client validation with server-side checks. The server can reject files after upload with custom error messages.

What you should see: the same rules enforced again on the server, where they actually count. Client checks are a convenience; this is the one an attacker cannot skip.

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Client-side rules catch obvious violations instantly. The server performs additional checks (virus scan, content analysis) and can return custom error messages.

<!-- Client-side validation catches issues early -->
<core-upload asp-upload-url="/api/upload/upload"
 asp-extensions=".jpg,.png,.gif,.pdf"
 asp-max-size="10MB"
 asp-multiple="true"
 asp-progress="true">
</core-upload>

// Server-side (C#) - return error to reject a file:
[HttpPost("upload")]
public IActionResult Upload(IFormFile file)
{
 if (!IsValid(file))
 return Json(new { error = "File rejected by server." });

 // Process file...
 return Json(new { success = true });
}