Okay I tried to avoid the 'technical term' of 'content disposition' which basically means "How is the content being disseminated/delivered to user agents..
meta http-equiv directives are for when the document meta data is being or has to be delivered in the form of HTTP headers.
However there are two solutions:
!; Use both and let the user agent choose which to apply.
2; Don't use the "old" format of XHTML. Use the current standard of HTML 5.
Also: "Transitional" document declarations are used to tell the 'consuming' UA that the document content does NOT necessarily conform 'strictly' to the defined DTD as the 'provider' is transitioning from one DTD to another. So, using solution 1 with a 'Transitional' form of DTD IS correct.
In essence you are using the idea of a DTD incorrectly. A DTD does NOT define the coding you use. It is actually the EXACT opposite. ... In that it is your coding that determines what DTD you should use.