v0.64.0 — Office files, spreadsheets and extra file types

Attachments that used to be refused without a word now go through, and hosts can accept formats of their own.

• Match by MIME type or extension. The browser takes File.type from the OS, which reports none for a .docx on a computer without Office (and for a .json on Windows): those files were refused and the file dialog did not even offer .docx. Every allowedFileTypes family now accepts by extension too. documents adds .odt and .rtf.
• New spreadsheets family (.xlsx, .xls, .xlsm, .ods, .csv), off by default, so no host changes behaviour on upgrade.
• additionalFileTypes?: string[] on ChatDrawer options and ChatInput: extra extensions ('.dwg', 'dxf') or MIME types, wildcards included ('application/zip', 'model/*'), on top of the families.
• customPromptBox receives fileAccept (for its <input accept>) and checkFile(file) (null | 'type' | 'size'), the same rules as the default composer.
• Refused files are reported above the input instead of only a console warning. Two new translatable texts: "{name}" cannot be attached: this file type is not allowed. and "{name}" is larger than {size} MB and cannot be attached.

Validated with 75 tests, typecheck, build, and a browser run of ChatInput inside a host app (.docx without MIME type attached; .xlsx refused unless spreadsheets is on; .exe refused; same run fails on 0.62.4). PR #72.