Here's what I'm thinking in terms of programmatic content limitations for my host, and likely default for future hosts. Feel free to express any concerns or comments you may have.
All fields will be UTF-8 encoded, all glyphs will be converted into their text variants. Following are the maximum values for various items: 128 posts per thread, and threads per board. 8 threads per page. 4 files per reply. 4Mb total upload size per reply. 4Kb for each message. 64bytes for the password and subject fields. Perhaps a 256byte minimum for thread messages and a 4byte minimum for thread subjects.
(anything with a text encoding), image/bmp, image/gif, image/jpeg, image/png, image/tiff, video/mp4, video/webm, video/ogg, audio/mpeg, audio/wav, audio/ogg, audio/mp4, audio/webm, audio/flac, audio/aac, audio/aacp, and application/pdf