在表单中上传文件需要使用HTML中的input标签，并设置type属性为file。例如：
<form action="upload.php" method="post" enctype="multipart/form-data">
<input type="file" name="fileToUpload" id="fileToUpload">
<input type="submit" value="Upload File" name="submit">
</form>其中，enctype属性指定了表单数据的编码类型，必须设置为multipart/form-data才能上传文件。
在服务器端，需要使用相应的编程语言（如PHP）处理上传的文件。可以通过$_FILES数组获取上传的文件信息，例如：
$target_dir = "uploads/";
$target_file = $target_dir . basename($_FILES["fileToUpload"]["name"]);
$uploadOk = 1;
$imageFileType = strtolower(pathinfo($target_file,PATHINFO_EXTENSION));
// Check if file already exists
if (file_exists($target_file)) {
echo "Sorry, file already exists.";
$uploadOk = 0;
}
// Check file size
if ($_FILES["fileToUpload"]["size"] > 500000) {
echo "Sorry, your file is too large.";
$uploadOk = 0;
}
// Allow certain file formats
if($imageFileType != "jpg" && $imageFileType != "png" && $imageFileType != "jpeg"
&& $imageFileType != "gif" ) {
echo "Sorry, only JPG, JPEG, PNG & GIF files are allowed.";
$uploadOk = 0;
}
// Check if $uploadOk is set to 0 by an error
if ($uploadOk == 0) {
echo "Sorry, your file was not uploaded.";
// if everything is ok, try to upload file
} else {
if (move_uploaded_file($_FILES["fileToUpload"]["tmp_name"], $target_file)) {
echo "The file ". htmlspecialchars( basename( $_FILES["fileToUpload"]["name"])). " has been uploaded.";
} else {
echo "Sorry, there was an error uploading your file.";
}
}在上述代码中，$target_dir变量指定了上传文件的目录，$target_file变量保存了上传文件的路径，$_FILES数组中的“fileToUpload”指定了上传文件的名称。最后使用move_uploaded_file函数将上传的文件从临时目录移动到指定目录中。