解决办法是把Axios扔进垃圾桶。我为这个问题挣扎了一天，这一天我再也回不来了。在https://github.com/axios/axios/issues/318上有一个很长的、有几年历史的帖子，人们抱怨无法让多部分表单上传与Axios一起工作。
我切换到request-promise模块，并使用以下简单代码在几分钟内使其工作：

const fs = require("fs-extra");
const rp = require('request-promise');
let out = await rp({
    method: 'POST',
    uri: 'http://mystrapihost/upload',
    formData: {
        // Like <input type="text" name="ref">
        'ref': "customer",  // name of the Strapi data type, singular
        'field': "attachments", // a field named "attachments" of type "Media"
        'refId': "838e238949ewhd82e8938299e289e99", // strapi ID of object to attach to
        // Like <input type="file" name="files">
        "files": {  // must be called "files" to be "seen" by Strapi Upload module
            name: "myfile.pdf",
            value: fs.createReadStream("/path/to/myfile.pdf"),
            options: {
                filename: "myfile.pdf",
                contentType: 'application/pdf'
            },
        },
    },
    headers: {Authorization: 'Bearer myjwtgobbledygook123456'}  // put your JWT code here
});
console.log(out);

好好享受吧！！

This is how I fixed it for Strapi v4

const onSubmitHandler = async (e) => {
        e.preventDefault();
        const formData = new FormData();

        formData.append('files', image);
        formData.append('ref', 'api::event.event');
        formData.append('refId', evtId);
        formData.append('field', 'image');

        const res = await fetch(`${API_URL}/api/upload`, {
            method: 'POST',
            body: formData,
        });

        if (res.ok) {
            imageUploaded();
        } else {
            console.log(res);
        }
    };