FileExtensionAttributeAdapter Fails jQuery Validation

[jditch]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

When using the FileExtensionAttribute on an IFormFile, the data-val-fileextensions-extensions attribute is not constructed per jQuery documentation https://jqueryvalidation.org/extension-method/. The client-side validation always fails because each file extension is prefixed with a period.

aspnetcore/src/Mvc/Mvc.DataAnnotations/src/FileExtensionsAttributeAdapter.cs

Lines 19 to 26 in bce4dd5

	// Build the extension list based on how the JQuery Validation's 'extension' method expects it
	// https://jqueryvalidation.org/extension-method/
	// These lines follow the same approach as the FileExtensionsAttribute.
	var normalizedExtensions = Attribute.Extensions.Replace(" ", string.Empty).Replace(".", string.Empty).ToLowerInvariant();
	var parsedExtensions = normalizedExtensions.Split(',').Select(e => "." + e);
	_formattedExtensions = string.Join(", ", parsedExtensions);
	_extensions = string.Join(",", parsedExtensions);

aspnetcore/src/Mvc/Mvc.DataAnnotations/src/FileExtensionsAttributeAdapter.cs

Line 36 in bce4dd5

MergeAttribute(context.Attributes, "data-val-fileextensions-extensions", _extensions);

jQuery validation expects the file extensions to be given as comma or pipe-delimited without periods or spaces so they can be matched using RegEx.
https://github.com/jquery-validation/jquery-validation/blob/6cd68f68e395b1c3a2588e6e7d64f561410c2dc0/src/additional/extension.js#L2-L4

The normalized list of comma-delimited extensions should be used for the attribute instead of _extensions.

aspnetcore/src/Mvc/Mvc.DataAnnotations/src/FileExtensionsAttributeAdapter.cs

Line 23 in bce4dd5

var normalizedExtensions = Attribute.Extensions.Replace(" ", string.Empty).Replace(".", string.Empty).ToLowerInvariant();

Expected Behavior

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Steps To Reproduce

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Exceptions (if any)

In jquery-validation 1.13.1 and older, this issue likely went mostly unnoticed due to a bug (This was fixed in jquery-validation 1.14.0)

.NET Version

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Anything else?

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