Laravel mimes validation always called even if no input file is uploaded (Not used required rule)

You can use the sometimes validation type for that.

'file' => 'sometimes|mimes:jpeg,png'

Validating When Present

In some situations, you may wish to run validation checks against a field only if that field is present in the input array. To quickly accomplish this, add the sometimes rule to your rule list.

Official Laravel docs

You can use sometimes rule.

'photo' => 'sometimes|mimes:jpeg,bmp,png'

You can check more about it here https://laravel.com/docs/5.4/validation#conditionally-adding-rules

I've issued the same problem recently with ajax uploading, and handled the file validation using

public function rules()
{
    $rules = [
        'invitation_message' => 'required'
    ];

    if (request()->hasFile('invitation_excel_sheet')) {
        $rules = array_merge($rules, [
            'invitation_excel_sheet' => 'mimes:xlsx,csv'
        ]);
    }

    return $rules;
}

Another way is to use exclude_if in model:

public function rules()
{
    return [
        'link' => 'sometimes',
        'newPdf' => 'exclude_if:link,null|mimes:pdf',
    ];
}