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Use this function to upload files and images to a web server application.
Note that the data is read from the contents of byte array and not from a file.
The fileName parameter is for telling the server what filename to use when saving the uploaded file.
If mimeType is not given and first 8 bytes of the data match PNG format header, then the
data is sent with "image/png" mimetype. Otherwise it is sent with "application/octet-stream"
mimetype.