I'm trying to POST a new media but get back the following error:

Invalid Content-Disposition supplied. Content-Disposition needs to be formatted as "filename=image.png" or similar.

I can work around that by using an invalid Content-Disposition of literally filename=unquoted_name.jpg, but that's not how the header is supposed to look like.

From RFC2616, section 19.5.1:

   content-disposition = "Content-Disposition" ":"
                         disposition-type *( ";" disposition-parm )
   disposition-type = "attachment" | disp-extension-token
   disposition-parm = filename-parm | disp-extension-parm
   filename-parm = "filename" "=" quoted-string
   disp-extension-token = token
   disp-extension-parm = token "=" ( token | quoted-string )

An example is

   Content-Disposition: attachment; filename="fname.ext"

Since RFC2616 describes headers in responses from a server, one could argue that this header isn't the same one and has other parsing rules, but I would then suggest that we use a dedicated X-Filename header instead of shadowing another well-known header.