Can't upload xlsx file to dropbox app

Yes, for these calls with the parameters in the header, you need to escape these characters. That is, when you use the “Dropbox-API-Arg” header, you need to make it “HTTP header safe”. This means using JSON-style “\uXXXX” escape codes for the character 0x7F and all non-ASCII characters.

Some, but not all, languages/libraries do this for you. For example, for JavaScript, to do this yourself you could do something like this:

var charsToEncode = /[\u007f-\uffff]/g;
function http_header_safe_json(v) {
 return JSON.stringify(v).replace(charsToEncode,
 function(c) { 
 return '\\u'+('000'+c.charCodeAt(0).toString(16)).slice(-4);
 }
 );
}

and then:

"Dropbox-API-Arg": http_header_safe_json(arg)