File upload not saving the file name in the database field

This below code is not updating the uploaded file name while updating but the same code we are using for creating new record it is working.

   public function update(Request $request, Account $account, Item $item)
    {

        $filename='';

        if( $request->hasFile('th_attach'))
            {
              $file = $request->file('th_attach');
              $ext = $file->getClientOriginalExtension();
              $filename = date('YmdHis').rand(1,99999).'.'.$ext;
              $file->storeAs('public/categories', $filename);
            }

           

        $account->th_attach = $filename;
        $account->th_supp_name = $request->th_supp_name;       

       $date  = Carbon::createFromFormat('d-m-Y', $request->th_bill_dt);        
       $account->th_bill_dt = $date;          
        
        $account->th_bill_no = $request->th_bill_no;
        $account->th_bill_amt = $request->th_bill_amt;
        $account->th_item_type = $request->th_item_type;
        $account->th_purpose = $request->th_purpose;        
        $account->save();
        
        return redirect()->route('admin.accounts.index')->with('info','Transaction updated successfully!');;
    }

Edit view

<div class="form-group">
  <div class = "row">
  
  <label class = "col-lg-1" for="">Attach Bill</label>
  <div class = "col-md-6">    
  <input type="file" id="validationCustom01" name="th_attach">
  <div class = "clear-fix"></div>
  </div>     
  </div>
  </div>
  

Can any one please help me?

Thank you