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Had a similar problem trying to send a text/csv file, fixed by parsing the content type before the route.
Like this:
app.use(bodyParser.raw({type: multipart/form-data}));
Before
app.use(multer({}))
bodyParser.raw([options]) returns middleware that parses all bodies as a Buffer and only looks at requests where the Content-Type header matches the type option
Source:stackexchange.com