Get filename from Content-Disposition header

getFilenameFromContentDisposition.ts

/**
* Support filename= and filename*= with the priority given to filename*=
* Encoding and locale can optionaly be provided with filename*=
* Return undefined if the content-disposition doesn't contain filename= or filename*=
*/
const { TextDecoder } = require("util");

function getFilenameFromContentDisposition(contentDisposition: string) {
  if (!contentDisposition) {
    return undefined
  }

  function findAndNormalizeParameter(parameter: string, contentDispositionParts: string[]) {
    return contentDispositionParts.find(n => n.toLowerCase().includes(`${parameter}=`))?.replace(/filename\*?=|"/gi, "")?.trim()
  }

  function decodeString(encodedString: string, encoding: string) {
    const uint8Array = new Uint8Array(encodedString.length);
    for (let i = 0; i < encodedString.length; i++) {
      uint8Array[i] = encodedString.charCodeAt(i)
    }
    try {
      const decoder = new TextDecoder(encoding.toLowerCase(), { fatal: true });
      return decoder.decode(uint8Array)
    } catch (typeError) {
      return undefined
    }
  }
  
  function readFilenameStar(contentDispositionParts: string[]) {
    const filenameStarParam = findAndNormalizeParameter("filename*", contentDispositionParts)
    if (!filenameStarParam) {
      return undefined
    }
    const parts = filenameStarParam.split("'")
    if (parts.length === 1) {
      return parts[0]
    }
    const [encoding, _, ...filenameParts] = parts
    const filename = filenameParts.join("")
    return ["utf-8", "utf8"].includes(encoding?.toLowerCase()) ? filename : decodeString(filename, encoding)
  }

  function readFilename(contentDispositionParts: string[]) {
    return findAndNormalizeParameter("filename", contentDispositionParts)
  }

  const contentDispositionParts = contentDisposition.split(";")
  const filename = readFilenameStar(contentDispositionParts) ?? readFilename(contentDispositionParts)
  return !filename ? undefined : decodeURIComponent(filename)
}

tests.ts

// Tests for the getFilenameFromContentDisposition function

it('should return the filename when the value of filename is wrapped in quotes', () => {
  const contentDisposition = 'form-data; name="offer"; filename="b968616e-bec7-4e21-8c01-502f9efa639e.pdf"';
  const result = getFilenameFromContentDisposition(contentDisposition);
  expect(result).toEqual('b968616e-bec7-4e21-8c01-502f9efa639e.pdf');
});

it('should return the filename when the value of filename is not wrapped in quotes', () => {
  const contentDisposition = 'form-data; name="offer"; filename=b968616e-bec7-4e21-8c01-502f9efa639e.pdf';
  const result = getFilenameFromContentDisposition(contentDisposition);
  expect(result).toEqual('b968616e-bec7-4e21-8c01-502f9efa639e.pdf');
});

it('should return the well decoded filename* when the value of filename* is wrapped in quotes', () => {
  const contentDisposition = 'form-data; name="offer"; filename*="ex%C3%A4mple.txt"';
  const result = getFilenameFromContentDisposition(contentDisposition);
  expect(result).toEqual('exämple.txt');
});

it('should return the well decoded filename* when UTF-8 encoding is provided and not wrapped in quotes', () => {
  const contentDisposition = 'form-data; name="offer"; filename*=UTF-8\'\'ex%C3%A4mple.txt';
  const result = getFilenameFromContentDisposition(contentDisposition);
  expect(result).toEqual('exämple.txt');
});

it('should return the filename when both filename and filename* are provided but filename* has an encoding that is not utf-8', () => {
  const contentDisposition = 'Inline; filename=example.txt; filename*=ISO-8859-1\'en\'fran\u00E9%C3%A7ois%20rod%C3%AEt%20%25%20bois%20%22\'%20(-.%24%5B%5D_%2B%40*%23%26%3D%3F*%2F).txt';
  const result = getFilenameFromContentDisposition(contentDisposition);
  expect(result).toEqual('franéçois rodît % bois " (-.$[]_+@*#&=?*/).txt');
});

it('should return undefined when only filename* is provided, but filename* has an encoding that is not utf-8', () => {
  const contentDisposition = 'Inline; filename*=ISO-8859-1\'en\'fran%C3%A7ois%20rod%C3%AEt%20%25%20bois%20%22\'%20(-.%24%5B%5D_%2B%40*%23%26%3D%3F*%2F).txt';
  const result = getFilenameFromContentDisposition(contentDisposition);
  expect(result).toEqual('françois rodît % bois " (-.$[]_+@*#&=?*/).txt');
});

it('should return the filename* when both filename and filename* are provided and the value of filename* is wrapped in quotes', () => {
  const contentDisposition = 'form-data; name="offer"; filename="test.txt"; filename*="ex%C3%A4mple.txt"';
  const result = getFilenameFromContentDisposition(contentDisposition);
  expect(result).toEqual('exämple.txt');
});

it('should return the filename* when both filename and filename* are provided but not in lowercase', () => {
  const contentDisposition = 'form-data; name="offer"; filename="test.txt"; FileName*="ex%C3%A4mple.txt"';
  const result = getFilenameFromContentDisposition(contentDisposition);
  expect(result).toEqual('exämple.txt');
});

it('should return undefined when undefined is given as parameter', () => {
  const result = getFilenameFromContentDisposition(undefined);
  expect(result).toEqual(undefined);
});

it('should return undefined when the content-disposition header has no filename or filename* parameter', () => {
  const contentDisposition = 'form-data; name="field1"';
  const result = getFilenameFromContentDisposition(contentDisposition);
  expect(result).toEqual(undefined);
});