Configure Content-Type header

[bjterry99]

Currently, there is no option to configure the Content-Type header when creating a message document.

When creating a message document with the following params, an Error - 11200 is thrown in the Twilio Error logs. This causes the media to not be sent correctly.

to: 'phoneNumber'
body: 'message body'
mediaUrl: [firebase storage media download URL]

In my experience I have tried to send a vCard stored in Firebase Storage. The vCard send correctly to iOS devices but Android devices to not receive the file correctly. Both cases throw the Error - 11200

Having the option to change the Content-Type header might fix the issue.