[FIX] ir.attachment: simplify xhtml-like mimetypes matching

Simplify/complements revision f6090e69e648d5891cb887e82fbfcbc868b574ac,
forcing text/plain for xml-like files.

Ignore forced `mimetype` when downloading attachments, if the model
has its own mimetype or if we can guess it.
`default_mimetype` still available as a fallback.

See also the list of mimetypes supported by common browsers, e.g.:

 - Firefox: CParserContext::SetMimeType in:
    https://hg.mozilla.org/firefox/file/default/parser/htmlparser/CParserContext.cpp

 - Chrome:  kSupportedNonImageTypes constant:
    https://cs.chromium.org/chromium/src/components/mime_util/mime_util.cc?sq=package:chromium&rcl=1467904650&l=89

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