Here is my crack at some real text. There are far too many things
people are trying to accomplish with this one mechanism. Below is a
list with the only restriction and only functionality I understand
these types to have.
The following are purposes people have argued for in content-type.
1) Top level content-type as an indicator of display required.
The type may or may not tell you anything about the type of display
you need. Any strict constrution of rules for types to accomplish
this results in awkward states. Even binary application files may be
viewed on a text only display if they are word processor files.
2) Top level content-type as an indicator of the transport encoding to
select either by the sending UA or by a gateway if conversion is
required.
Content type has NO bearing on the selection of a transport encoding.
Selection of a transport encoding based in a type in no way implies a
more optimal encoding scheme. See previous messages on use of Quoted
printable with TEXT in Cyrillic, Greek, or Arabic 8859 and Base 64
with BINARY where the content is a word processor file 95% 7 bit text
and mostly obeying the line length conventions.
Greg V.
----- Suggested text for content types --------------
TEXT: Requires a text oriented display only. No software is required to
get the full meaning of the text. It is full revisable.
(Unicode is clearly text, but in current form it requires
Binary transport)
TEXT PLUS: Requires a text oriented display only. Application software
may enhance the appearance of the text, but IS NOT REQUIRED to
gather the general idea of the message. The message is fully
revisable by a knowledgeable operator without use of an
application program.
(I understand this to include LaTeX, .nroff, and richtext,
which are usable and revisable on an character only display
even if the "pretty" output is not viewable. Note: I cannot
read and understand postscript in the manner in which I
understand this type. Do not expect me to understand
PostScript without an interpreter.)
MESSAGE: Implies nothing about the display. Contains a fully formated
822 conformant message which may contain any combination of
other content-types.
MULTIPART: This contents uses a special mechanism for indicating
multiple content-types in a single message. This type implies
nothing about the display, and may have sections containing any
other content-type.
AUDIO: This message contains information which requires a speaker to
"display". It is not possible to interpret this messsage in
any reasonable fashion without a speaker. It implies nothing
about the revisability of the message.
(I do not see phonetic synthizer input in this category, but
rather in Application)
IMAGE: Requires a bit-mapped display to view the essential information
in this message. This does not imply that the image is
revisable nor requires a SPECIFIC application to be seen, i.e.
any generic TIFF viewer will work.)
(I see TIFF, GIFF, G3 FAX and maybe even old-style Postscript.
I can not revise postscript nor read it in any meaningful way
without a bitmap display or printer)
VIDEO: Required a bit-mapped display and a lot of disk space and a
lot of CPU and maybe even signal processing hardware. It
implies nothing about being revisble.
BINARY: Is not intended to be displayed in any fashion to the user.
This is use for program code, directory dumps, and other data
which is not explicitly input to a program but rather to be
used in its native form. The most rational thing to do with
this information is to write it to a file at the users request.
APPLICATION: This is information which must be processed by an
application before it is viewable or usable to a user. It is
explicitly revisable. This implies nothing about the nature
of the user display to be used.
(This is where I would put Word Perfect files, Framemaker
data {all forms}, and Lotus spreadsheets. If PostScript is
explicity revisable, it can go here, otherwise it is an IMAGE
form)