Blazor DragAndDropFileInput

<DragAndDropFileInput HintText="Drop files here on desktop, or tap to browse on touch devices." />

<DragAndDropFileInput Multiple="true"
                      MaxFileCount="3"
                      MaxFileSize="1048576"
                      AllowedFileTypes="@allowedFileTypes"
                      InvalidFileTypeErrorMessage="{FileName} must use one of: {AllowedFileTypes}."
                      MaxFileCountErrorMessage="{FileName} would exceed the {MaxFileCount}-file limit."
                      MaxFileSizeErrorMessage="{FileName} exceeds the {MaxFileSize}-byte limit."
                      SingleFileErrorMessage="Select only one file."
                      HintText="Select up to three PDF or PNG files, each no larger than 1 MB." />

@code {
    private readonly string[] allowedFileTypes = [".pdf", "png"];
}

@using System.ComponentModel.DataAnnotations
@using Microsoft.AspNetCore.Components.Forms

<EditForm EditContext="@editContext" OnValidSubmit="HandleValidSubmit">
    <DataAnnotationsValidator />
    <ValidationSummary />

    <div class="mb-3">
        <label class="form-label">Supporting document <span class="text-danger">*</span></label>
        <DragAndDropFileInput AllowedFileTypes="@allowedFileTypes"
                              MaxFileSize="1048576"
                              FilesChanged="OnFilesChanged"
                              HintText="Drop a PDF up to 1 MB, or tap to browse." />
        <ValidationMessage For="@(() => upload.DocumentName)" />
    </div>

    <Button Type="ButtonType.Submit" Color="ButtonColor.Primary">Submit</Button>

    @if (submitted)
    {
        <div class="alert alert-success mt-3" role="status">Form submitted with @upload.DocumentName.</div>
    }
</EditForm>

@code {
    private readonly string[] allowedFileTypes = ["pdf"];
    private readonly UploadModel upload = new();
    private EditContext editContext = null!;
    private bool submitted;

    protected override void OnInitialized()
    {
        editContext = new EditContext(upload);
    }

    private void HandleValidSubmit() => submitted = true;

    private void OnFilesChanged(IReadOnlyList<IBrowserFile> files)
    {
        upload.DocumentName = files.FirstOrDefault()?.Name;
        submitted = false;
        editContext.NotifyFieldChanged(new FieldIdentifier(upload, nameof(upload.DocumentName)));
    }

    private sealed class UploadModel
    {
        [Required(ErrorMessage = "A supporting document is required.")]
        public string? DocumentName { get; set; }
    }
}