How to make the form file not required if the path to files already exists in the DB - asp.net-core

Question

I have a view model set up as:
public class BookViewModel
{
public int Id { get; set; }
[Display(Name = "Book Name")]
public string Name { get; set; }
public string Path { get; set; }
[Display(Name = "Pdf Book")]
public IFormFile BookPdfFile { get; set; }
}
In the view, I am using the view model properties
<form asp-action="CreateEdit" method="post" enctype="multipart/form-data">
<div asp-validation-summary="ModelOnly" class="text-danger"></div>
<input asp-for="Id" type="hidden" />
<div class="form-group">
<label asp-for="Name" class="control-label"></label>
<input asp-for="Name" class="form-control" />
<span asp-validation-for="Name" class="text-danger"></span>
</div>
#if (!string.IsNullOrEmpty(Model.BookPath))
{
<div class="form-group">
<label>Uploaded Book:</label>
<a asp-action="Download" asp-route-id="#Model.Id">#Model.Name</a>
</div>
}
<div class="form-group">
<label asp-for="BookPdfFile" class="control-label"></label>
<input class="form-control" type="file" asp-for="BookPdfFile" accept=".pdf">
<span asp-validation-for="BookPdfFile" class="text-danger"></span>
</div>
</form>
Using this View model in the view in this way always asks for the file to be uploaded. If the file is uploaded during the create mode or if it has the file path, how to not ask to make the form file required?
The Book entity is:
public class Book
{
public int Id { get; set; }
[MaxLength(100)]
public string Title { get; set; }
[MaxLength(50)]
public string? Path { get; set; }
public ICollection<BookChapter> Chapters { get; set; }
}
How could I modify my code to make the form file not required if the path already exists in the DB?
On the server side I could add the validation attribute but how would I make that work on the client side as well?
I have yet not worked on posting and saving the files to the controller on HttpPost. So, my controller is just a one-line initializing the view model.
public async Task<IActionResult> CreateBook()
{
BookViewModel bookVM = new BookViewModel();
return View("CreateEdit", bookVM);
}