A proposal for optional error handling in processing the Content-Disposition header field

Extracting Parameter Values From Header Fields

To extract the value for a given parameter-name from an unparsed-string, parse the unparsed-string using the following grammar:

unparsed-string = *OCTET name *LWS "=" value [ ";" *OCTET ]
value           = <OCTET, except ";">

where the name production is a gramatical production that is a case-insensitive match for the given parameter-name. If the unparsed-string can be parsed by the grammar in multple ways, choose the one in which name appears as close to the beginning of the string as possible. If the unparsed-string cannot be parsed by the grammar above, return the empty string.

Decoding the File Name

To filename-decode an encoded-string, parse the encoded-string using the following grammar:

encoded-string = word *( 1*delimiter word )
delimiter      = LWS
word           = <OCTET, except delimiter>

Consider each gramatical element (either a delimiter or a word) in the order they appear in the encoded-string:

1. If the gramatical element is a delimiter, emit a SP character.
2. If the gramatical element is a word, process the element as follows:
   1. If the word contains non-ASCII characters, emit the word (decoded as ISO-8859-1) and proceed to the next gramatical element.
   2. Let the url-unescaped-word be the word %-unescaped.
   3. Emit the url-unescaped-word (decoded as UTF-8) and proceed to the next grammatical element. (There's actually more sadness here if the url-unescaped-word isn't valid UTF-8.)

The emitted characters are the decoded file name.

Determining the File Name

To determine the file name indicated by a Content-Disposition header field, use the following algorithm:

1. Let filename-star be the value extracted from the Content-Disposition header field for for the "filename*" parameter.
2. If filename-star parses as a RFC5987-value, return the RFC5987-value of filename-star and abort these steps.
3. Let filename be the value extracted from the Content-Disposition header field for the "filename" parameter.
4. If filename is empty, instead let filename be the value extracted from the Content-Disposition header field for the "name" parameter.
5. If filename is empty, return the empty string and abort these steps.
6. Return the filename-decoding of filename.

Determining the Disposition

To determine the disposition-type, parse the Content-Disposition header field using the following grammar:

unparsed-string  = *LWS nominal-type *OCTET
nominal-type = "inline" / "filename" / "name" / ";"

If the Content-Disposition header field is non-empty and fails to parse, then the disposition type is "attachment". Otherwise, the disposition-type is "inline".

Processing the Content-Disposition Header Field

To process the Content-Disposition header field, use the following algorithm:

1. Determine the disposition-type.
2. If the disposition-type is "inline", then ...
3. If the disposition-type is "attachment", then let filename be the file name indicated by the header field. ...