Hi,vkagrawal

By using bruce's link, I made a demo for you to refer, if you have any questions, welcome to post your questions:

CustomTagHelper:

[HtmlTargetElement("input", Attributes = "asp-for")]
    public class CustomTagHelper : TagHelper
    {
        public override int Order { get; } = int.MaxValue;

        [HtmlAttributeName("asp-for")]
        public ModelExpression For { get; set; }

        public override void Process(TagHelperContext context, TagHelperOutput output)
        {
            base.Process(context, output);

            if (context.AllAttributes["data-maxsize"] == null)
            {
                var maxLength = GetMaxLength(For.ModelExplorer.Metadata.ValidatorMetadata);
                if (maxLength > 0)
                {
                    output.Attributes.Add("data-maxsize", maxLength);
                }
            }
            if (context.AllAttributes["accept"] == null)
            {
                string[] allowed = Getallowed(For.ModelExplorer.Metadata.ValidatorMetadata);
                if (allowed !=null)
                {
                    string allowed1 = string.Join(",", allowed);     //convert array to string
                    output.Attributes.Add("accept",allowed1);
                }
            }
        }

        private static int GetMaxLength(IReadOnlyList<object> validatorMetadata)
        {
            for (var i = 0; i < validatorMetadata.Count; i++)
            {              
                if (validatorMetadata[i] is MaxFileSizeAttribute maxFileSizeAttribute && maxFileSizeAttribute.maxFileSizeInBytes > 0)
                {
                    return maxFileSizeAttribute.maxFileSizeInBytes;
                }              
            }
            return 0;
        }
        private static string[] Getallowed(IReadOnlyList<object> validatorMetadata)
        {
            for (var i = 0; i < validatorMetadata.Count; i++)
            {
                if (validatorMetadata[i] is AllowedExtensionsAttribute allowedExtensionsAttribute && allowedExtensionsAttribute.allowedExtensions!=null)
                {
                    return allowedExtensionsAttribute.allowedExtensions;
                }
            }
            return null;
        }
    }

CustomAttribute class (almost the same like another):

public class AllowedExtensionsAttribute : ValidationAttribute
    {
        public string[] allowedExtensions;
        public AllowedExtensionsAttribute(string[] allowedExtensions)
        {
            this.allowedExtensions = allowedExtensions;
        }
    }

Result:

Best Regards,

Jerry Cai