什么可能导致这种形式的问题?
<form enctype="multipart/form-data" action="" method="post">
    <p>
        <label>Please select a picure to upload:</label><br />
        <input type="file" name="file" />
    </p>
    <p>
        <input type="submit" name="button" id="button" value="Upload Picture" />
    </p>
</form>
文件作为字符串而不是文件提交...
var_dump($_POST):
array (size=2)
    'file' => string 'Visit Card.png' (length=14)
    'button' => string 'Upload Picture' (length=14)
var_dump($_FILES):
array (size=0)
    empty