Added 'hashAlgorithm' options to use another algorithm for hashing file content other than md5

[remtori]

Like the title said, this PR added 'hashAlgorithm' options to specify other hash algorithm than md5.

In my use case, i need sha1 content hash instead of md5 so i will use its like:

const fileUpload = require('express-fileupload');

// omited

app.use(fileUpload({ 
    useTempFiles: true,
    hashAlgorithm: 'sha1',
});

app.post('/upload', (req, res) => {
    console.log('File content sha1 hash: ' + req.files.file.sha1);
    res.send('Ok');
});

[imjordanxd]

couldn't we just use hash here as the object key?

[remtori]

yes, but that a breaking change require a major version bump.
doing its this way make it backward compatiable with existing code expecting the md5 field

[Cerothen]

I'd be a big fan if this PR got merged and released as I too would like to use a hash algorithm that is less likely to have a collision.

As noted in another PR the test that failed is also failing in the master build and isn't related to the changes. #253 (comment)