Vue2+Laravel8中Axios更新数据时File字段未更新问题

提交表单后，其他数据均可正常更新，但文件字段始终无法传入请求。已在表单中添加enctype="multipart/form-data"，但控制器检测不到文件存在，甚至返回了!$request->file()的响应，说明文件根本没传送到后端。以下是相关代码：

前端表单代码 #

<b-form @submit.prevent="update" enctype="multipart/form-data">
...
<b-form-file
    v-model="invoice.file"
    placeholder="Choose a file or drop it here..."
    drop-placeholder="Drop file here..."
    v-on:change="onChange"
/>
</b-form>

前端脚本代码 #

data() {
return {
  invoice: {
    ...
    file: "",
    _method: "patch"
  }
};
},

methods: {
onChange(e) {
  this.file = e.target.files[0];
},

async update() {
  await this.axios
    .post(`/api/auth/outstanding-payment/${this.$route.params.id}`, this.invoice)
    .then((response) => {
      this.$router.push({ name: "apps-invoice-list" });
    })
    .catch((error) => {
      console.log(error);
    });
},

Laravel控制器代码 #

public function update(Request $request, OutstandingPayment $outstandingPayment)
{
    $payment_voucher_supporting_doc = '';

    if ($request->hasFile('file')) {
        $payment_voucher_supporting_doc = time() . '.' . $request->file->extension();
        $request->file->storeAs('uploads/images/test', $payment_voucher_supporting_doc);

        if ($outstandingPayment->payment_voucher_supporting_doc) {
            Storage::delete('public/images/' . $outstandingPayment->payment_voucher_supporting_doc);
        }
    } else {
        $payment_voucher_supporting_doc = $outstandingPayment->payment_voucher_supporting_doc;
    }

    $postData = [
        'payment_voucher_file_ref_no' => $request->payment_voucher_file_ref_no,
        'payment_voucher_date' => $request->payment_voucher_date,
        ...
        'payment_voucher_received_by' => $request->payment_voucher_received_by,
        'payment_voucher_supporting_doc' => $payment_voucher_supporting_doc,
    ];

    $outstandingPayment->update($postData);
}

排查补充 #

修改控制器添加文件检测逻辑：

if(!$request->file()) {
    return response()->json("Hi!");
}

提交表单后收到了"Hi!"的响应，确认文件未传入请求中。

请问问题出在哪里？

内容的提问来源于stack exchange，提问作者Sam