如何在上传前用javascript检查文件MIME类型？

Question

我已阅读这个和这个问题,这似乎表明,文件的MIME类型可以使用在客户端的JavaScript进行检查.现在,我知道真正的验证仍然必须在服务器端完成.我想执行客户端检查以避免不必要的服务器资源浪费.

为了测试是否可以在客户端完成,我将JPEG测试文件的扩展名更改为.png并选择要上载的文件.在发送文件之前,我使用javascript控制台查询文件对象:

document.getElementsByTagName('input')[0].files[0];

这是我在Chrome 28.0上获得的:

File {webkitRelativePath:"",lastModifiedDate:Tue Oct 16 2012 10:00:00 GMT + 0000(UTC),name:"test.png",输入:"image/png",大小:500055 ......}

它显示的类型image/png似乎表明检查是基于文件扩展名而不是MIME类型完成的.我尝试了Firefox 22.0,它给了我相同的结果.但是根据W3C规范,应该实现MIME Sniffing.

我是否可以说目前无法使用javascript检查MIME类型？或者我错过了什么？

Answer 1

FileReader在将其上载到服务器之前,您可以使用JavaScript轻松确定文件MIME类型.我同意我们应该优先考虑客户端的服务器端检查,但仍然可以进行客户端检查.我将向您展示如何在底部提供工作演示.

检查您的浏览器是否支持File和Blob.所有主要的应该.

if (window.FileReader && window.Blob) {
    // All the File APIs are supported.
} else {
    // File and Blob are not supported
}

步骤1:

您可以File从<input>像这样的元素(ref)中检索信息:

<input type="file" id="your-files" multiple>
<script>
var control = document.getElementById("your-files");
control.addEventListener("change", function(event) {
    // When the control has changed, there are new files
    var files = control.files,
    for (var i = 0; i < files.length; i++) {
        console.log("Filename: " + files[i].name);
        console.log("Type: " + files[i].type);
        console.log("Size: " + files[i].size + " bytes");
    }
}, false);
</script>

这是上面的一个拖放版本(参考):

<div id="your-files"></div>
<script>
var target = document.getElementById("your-files");
target.addEventListener("dragover", function(event) {
    event.preventDefault();
}, false);

target.addEventListener("drop", function(event) {
    // Cancel default actions
    event.preventDefault();
    var files = event.dataTransfer.files,
    for (var i = 0; i < files.length; i++) {
        console.log("Filename: " + files[i].name);
        console.log("Type: " + files[i].type);
        console.log("Size: " + files[i].size + " bytes");
    }
}, false);
</script>

第2步:

我们现在可以检查文件并梳理标头和MIME类型.

✘快速方法

您可以天真地向Blob询问使用此模式表示的任何文件的MIME类型:

var blob = files[i]; // See step 1 above
console.log(blob.type);

对于图像,MIME类型返回如下:

image/jpeg
image/png
...

警告:从文件扩展名中检测到MIME类型,可以被欺骗或欺骗.可以将a重命名.jpg为a .png,并将MIME类型报告为image/png.

✓正确的标题检查方法

为了获得客户端文件的bonafide MIME类型,我们可以更进一步,检查给定文件的前几个字节,以便与所谓的幻数进行比较.请注意,这并不完全是直截了当的,因为例如,JPEG有一些"神奇的数字".这是因为自1991年以来格式已经发展.您可能只检查前两个字节,但我更喜欢检查至少4个字节以减少误报.

JPEG的示例文件签名(前4个字节):

FF D8 FF E0(SOI + ADD0)
FF D8 FF E1(SOI + ADD1)
FF D8 FF E2(SOI + ADD2)

以下是检索文件头的基本代码:

var blob = files[i]; // See step 1 above
var fileReader = new FileReader();
fileReader.onloadend = function(e) {
  var arr = (new Uint8Array(e.target.result)).subarray(0, 4);
  var header = "";
  for(var i = 0; i < arr.length; i++) {
     header += arr[i].toString(16);
  }
  console.log(header);

  // Check the file signature against known types

};
fileReader.readAsArrayBuffer(blob);

然后,您可以像这样确定真实的MIME类型(此处和此处有更多文件签名):

switch (header) {
    case "89504e47":
        type = "image/png";
        break;
    case "47494638":
        type = "image/gif";
        break;
    case "ffd8ffe0":
    case "ffd8ffe1":
    case "ffd8ffe2":
    case "ffd8ffe3":
    case "ffd8ffe8":
        type = "image/jpeg";
        break;
    default:
        type = "unknown"; // Or you can use the blob.type as fallback
        break;
}

根据预期的MIME类型,根据需要接受或拒绝文件上传.

演示

这是一个本地文件和远程文件的工作演示(我不得不绕过CORS只是为了这个演示).打开片段,运行它,您应该看到显示三种不同类型的远程图像.在顶部,您可以选择本地图像或数据文件,并将显示文件签名和/或MIME类型.

请注意,即使重命名了图像,也可以确定其真正的MIME类型.见下文.

截图

// Return the first few bytes of the file as a hex string
function getBLOBFileHeader(url, blob, callback) {
  var fileReader = new FileReader();
  fileReader.onloadend = function(e) {
    var arr = (new Uint8Array(e.target.result)).subarray(0, 4);
    var header = "";
    for (var i = 0; i < arr.length; i++) {
      header += arr[i].toString(16);
    }
    callback(url, header);
  };
  fileReader.readAsArrayBuffer(blob);
}

function getRemoteFileHeader(url, callback) {
  var xhr = new XMLHttpRequest();
  // Bypass CORS for this demo - naughty, Drakes
  xhr.open('GET', '//cors-anywhere.herokuapp.com/' + url);
  xhr.responseType = "blob";
  xhr.onload = function() {
    callback(url, xhr.response);
  };
  xhr.onerror = function() {
    alert('A network error occurred!');
  };
  xhr.send();
}

function headerCallback(url, headerString) {
  printHeaderInfo(url, headerString);
}

function remoteCallback(url, blob) {
  printImage(blob);
  getBLOBFileHeader(url, blob, headerCallback);
}

function printImage(blob) {
  // Add this image to the document body for proof of GET success
  var fr = new FileReader();
  fr.onloadend = function() {
    $("hr").after($("<img>").attr("src", fr.result))
      .after($("<div>").text("Blob MIME type: " + blob.type));
  };
  fr.readAsDataURL(blob);
}

// Add more from http://en.wikipedia.org/wiki/List_of_file_signatures
function mimeType(headerString) {
  switch (headerString) {
    case "89504e47":
      type = "image/png";
      break;
    case "47494638":
      type = "image/gif";
      break;
    case "ffd8ffe0":
    case "ffd8ffe1":
    case "ffd8ffe2":
      type = "image/jpeg";
      break;
    default:
      type = "unknown";
      break;
  }
  return type;
}

function printHeaderInfo(url, headerString) {
  $("hr").after($("<div>").text("Real MIME type: " + mimeType(headerString)))
    .after($("<div>").text("File header: 0x" + headerString))
    .after($("<div>").text(url));
}

/* Demo driver code */

var imageURLsArray = ["http://media2.giphy.com/media/8KrhxtEsrdhD2/giphy.gif", "http://upload.wikimedia.org/wikipedia/commons/e/e9/Felis_silvestris_silvestris_small_gradual_decrease_of_quality.png", "http://static.giantbomb.com/uploads/scale_small/0/316/520157-apple_logo_dec07.jpg"];

// Check for FileReader support
if (window.FileReader && window.Blob) {
  // Load all the remote images from the urls array
  for (var i = 0; i < imageURLsArray.length; i++) {
    getRemoteFileHeader(imageURLsArray[i], remoteCallback);
  }

  /* Handle local files */
  $("input").on('change', function(event) {
    var file = event.target.files[0];
    if (file.size >= 2 * 1024 * 1024) {
      alert("File size must be at most 2MB");
      return;
    }
    remoteCallback(escape(file.name), file);
  });

} else {
  // File and Blob are not supported
  $("hr").after( $("<div>").text("It seems your browser doesn't support FileReader") );
} /* Drakes, 2015 */

img {
  max-height: 200px
}
div {
  height: 26px;
  font: Arial;
  font-size: 12pt
}
form {
  height: 40px;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<form>
  <input type="file" />
  <div>Choose an image to see its file signature.</div>
</form>
<hr/>

Answer 2

如其他答案中所述,您可以通过检查文件的第一个字节中的文件签名来检查mime类型.

但其他答案正在做的是将整个文件加载到内存中以检查签名,这非常浪费,如果您偶然选择一个大文件,可能会轻易冻结您的浏览器.

/**
 * Load the mime type based on the signature of the first bytes of the file
 * @param  {File}   file        A instance of File
 * @param  {Function} callback  Callback with the result
 * @author Victor www.vitim.us
 * @date   2017-03-23
 */
function loadMime(file, callback) {
    
    //List of known mimes
    var mimes = [
        {
            mime: 'image/jpeg',
            pattern: [0xFF, 0xD8, 0xFF],
            mask: [0xFF, 0xFF, 0xFF],
        },
        {
            mime: 'image/png',
            pattern: [0x89, 0x50, 0x4E, 0x47],
            mask: [0xFF, 0xFF, 0xFF, 0xFF],
        }
        // you can expand this list @see https://mimesniff.spec.whatwg.org/#matching-an-image-type-pattern
    ];

    function check(bytes, mime) {
        for (var i = 0, l = mime.mask.length; i < l; ++i) {
            if ((bytes[i] & mime.mask[i]) - mime.pattern[i] !== 0) {
                return false;
            }
        }
        return true;
    }

    var blob = file.slice(0, 4); //read the first 4 bytes of the file

    var reader = new FileReader();
    reader.onloadend = function(e) {
        if (e.target.readyState === FileReader.DONE) {
            var bytes = new Uint8Array(e.target.result);

            for (var i=0, l = mimes.length; i<l; ++i) {
                if (check(bytes, mimes[i])) return callback("Mime: " + mimes[i].mime + " <br> Browser:" + file.type);
            }

            return callback("Mime: unknown <br> Browser:" + file.type);
        }
    };
    reader.readAsArrayBuffer(blob);
}


//when selecting a file on the input
fileInput.onchange = function() {
    loadMime(fileInput.files[0], function(mime) {

        //print the output to the screen
        output.innerHTML = mime;
    });
};

<input type="file" id="fileInput">
<div id="output"></div>

Answer 3

For anyone who's looking to not implement this themselves, Sindresorhus has create a utility that works in the browser and has the header-to-mime mappings for most documents you could want.

https://github.com/sindresorhus/file-type

You could combine Vitim.us's suggestion of only reading in the first X bytes to avoid loading everything into memory with using this utility (example in es6):

import fileType from 'file-type'; // or wherever you load the dependency

const blob = file.slice(0, fileType.minimumBytes);

const reader = new FileReader();
reader.onloadend = function(e) {
  if (e.target.readyState !== FileReader.DONE) {
    return;
  }

  const bytes = new Uint8Array(e.target.result);
  const { ext, mime } = fileType.fromBuffer(bytes);

  // ext is the desired extension and mime is the mimetype
};
reader.readAsArrayBuffer(blob);

Answer 4

这是一个支持 webp 的 Typescript 实现。这是基于 Vitim.us 的 JavaScript 答案。

interface Mime {
  mime: string;
  pattern: (number | undefined)[];
}

// tslint:disable number-literal-format
// tslint:disable no-magic-numbers
const imageMimes: Mime[] = [
  {
    mime: 'image/png',
    pattern: [0x89, 0x50, 0x4e, 0x47]
  },
  {
    mime: 'image/jpeg',
    pattern: [0xff, 0xd8, 0xff]
  },
  {
    mime: 'image/gif',
    pattern: [0x47, 0x49, 0x46, 0x38]
  },
  {
    mime: 'image/webp',
    pattern: [0x52, 0x49, 0x46, 0x46, undefined, undefined, undefined, undefined, 0x57, 0x45, 0x42, 0x50, 0x56, 0x50],
  }
  // You can expand this list @see https://mimesniff.spec.whatwg.org/#matching-an-image-type-pattern
];
// tslint:enable no-magic-numbers
// tslint:enable number-literal-format

function isMime(bytes: Uint8Array, mime: Mime): boolean {
  return mime.pattern.every((p, i) => !p || bytes[i] === p);
}

function validateImageMimeType(file: File, callback: (b: boolean) => void) {
  const numBytesNeeded = Math.max(...imageMimes.map(m => m.pattern.length));
  const blob = file.slice(0, numBytesNeeded); // Read the needed bytes of the file

  const fileReader = new FileReader();

  fileReader.onloadend = e => {
    if (!e || !fileReader.result) return;

    const bytes = new Uint8Array(fileReader.result as ArrayBuffer);

    const valid = imageMimes.some(mime => isMime(bytes, mime));

    callback(valid);
  };

  fileReader.readAsArrayBuffer(blob);
}

// When selecting a file on the input
fileInput.onchange = () => {
  const file = fileInput.files && fileInput.files[0];
  if (!file) return;

  validateImageMimeType(file, valid => {
    if (!valid) {
      alert('Not a valid image file.');
    }
  });
};

<input type="file" id="fileInput">

Answer 5

如果您只想检查上传的文件是否为图像，您可以尝试将其加载到<img>标签中以检查是否有任何错误回调。

例子：

var input = document.getElementsByTagName('input')[0];
var reader = new FileReader();

reader.onload = function (e) {
    imageExists(e.target.result, function(exists){
        if (exists) {

            // Do something with the image file.. 

        } else {

            // different file format

        }
    });
};

reader.readAsDataURL(input.files[0]);


function imageExists(url, callback) {
    var img = new Image();
    img.onload = function() { callback(true); };
    img.onerror = function() { callback(false); };
    img.src = url;
}

Answer 6

这是你必须做的

var fileVariable =document.getElementsById('fileId').files[0];

如果要检查图像文件类型，则

if(fileVariable.type.match('image.*'))
{
 alert('its an image');
}