add content type to buffer

[cr24osome]

If you try to send a document to xero , the file gets uploaded successfully but size is different and It doesn't display the content but end up with an error message. It seems that the content type is missing from the buffer upload. This is a suggested fix

[gierschv]

+1 for this, this patch is badly indented, though, and this breaks the compatibility so this new parameter should be an option.