这是你必须要做的

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

如果你想检查图像文件类型，那么

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

这是一个最小的typescript/promise util浏览器;

export const getFileHeader = (file: File): Promise<string> => {
  return new Promise(resolve => {
    const headerBytes = file.slice(0, 4); // Read the first 4 bytes of the file
    const fileReader = new FileReader();
    fileReader.onloadend = (e: ProgressEvent<FileReader>) => {
      const arr = new Uint8Array(e?.target?.result as ArrayBufferLike).subarray(
        0,
        4,
      );
      let header = '';
      for (let i = 0; i < arr.length; i++) {
        header += arr[i].toString(16);
      }
      resolve(header);
    };
    fileReader.readAsArrayBuffer(headerBytes);
  });
};

在你的验证中像这样使用(我需要一个PDF检查);

// https://mimesniff.spec.whatwg.org/#matching-an-image-type-pattern
const pdfBytePattern = "25504446"
const fileHeader = await getFileHeader(file)
const isPdf = fileHeader === pdfBytePattern // => true

下面是一个支持webp的Typescript实现。 这是基于vic .us的JavaScript回答。

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.');
    }
  });
};

<输入类型=“文件” id=“文件输入”>

正如在其他答案中所述，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。

但是其他答案所做的是将整个文件加载到内存中以检查签名，这是非常浪费的，并且如果您无意中选择了一个大文件，很容易冻结您的浏览器。

/** * 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>

对于那些不希望自己实现这个功能的人来说，Sindresorhus创建了一个在浏览器中工作的实用程序，并为您想要的大多数文档提供了头到mime的映射。

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

你可以结合受害者。us的建议是只读取前X字节，以避免使用这个实用程序将所有内容加载到内存中(es6中的示例):

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);

这是你必须要做的

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

如果你想检查图像文件类型，那么

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