How to make File Upload a required field

I have a file upload input where I want to make it a required field in my view. How do I do that. I assume you can't do it as an attribute in the view model?

View:

<div class="form-group col-md-8"><div class="form-group col-md-8"><div asp-validation-summary="All" id="validation-error" class="text-danger custom-validation-summary"></div></div><label class="control-label">Document Type</label>
    @Html.DropDownListFor(m => m.DocumentType,      
        Model.DocumentTypes,        "- Please select a document type -",        
        new { @class = "form-control" })</div><div class="form-group col-md-8"><label class="control-label">Title</label><input id="document-title" asp-for="@Model.Document.Title" class="form-control" /></div><div class="form-group col-md-8"><label class="control-label">Upload file:</label></div><div class="form-group col-md-8">
    @{<input id="document-item-file-upload" type="file" name="files" />
    }</div><div class="form-group col-md-8 col-lg-4"><label class="control-label">Date </label><div class="input-group date" id="document-date" data-target-input="nearest"><input type="text" class="form-control" data-target="#document-date"
               asp-for="@Model.Document.Date" /><span class="input-group-addon" data-target="#document-date" data-toggle="datetimepicker"><span class="fa fa-calendar"></span></span></div></div><div class="form-group col-md-12"><button id="add-another-document"
            type="button"
            class="btn btn-forum col-sm-12 col-md-7 col-lg-4">Add & Create Another</button><input id="document-submit" type="button"
           value="Add & Return to Documents"
           class="btn btn-forum col-sm-12 col-md-7 col-lg-4" /><button id="document-cancel"
            class="cancel btn btn-forum col-sm-12 col-md-8 col-lg-3"
            type="button">Return to Documents</button></div>

View Model:

public class CreateEditDocumentViewModel
    {
        public int DocumentId { get; set; }
        public string Title { get; set; }
        public int FileId { get; set; }

        [Required(ErrorMessage = "Please select a document type")]
        public string FileName { get; set; }

        public EgendaFile EgendaFile { get; set; }

        public bool IsAddAnother { get; set; } 
    }
}