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  = unbalanced-block / block * ( ";" block ) [ ";" unbalanced-block ]
block            = *run
unbalanced-block = *run unbalanced-run
run              = unquoted-run / quoted-run
unquoted-run     = non-quote *boring-octet
quoted-run       = <"> *non-quote <">
unbalanced-run   = <"> *non-quote
non-quote        = <OCTET, except <"> >
boring-octet     = <OCTET, except <"> and ";">

Parse each block, in turn, (including the unbalanced-block, if present) using the following grammar:

block = *LWS name *LWS "=" value
value = OCTET

where the name production is a gramatical production that is a case-insensitive match for the given parameter-name. If any block can be parsed by the grammar, let the raw-value be the characters produced by the value production of the first such block. Otherwise, let the raw-value be the empty string.

If the raw-value both begins an ends with a <"> character, return the value stripped of those <"> characters. Otherwise return the raw-value.

Decoding the File Name

To filename-decode an encoded-string, use the following algorithm:

1. If the encoded-string contains non-ASCII characters, emit the encoded-string (decoded as ISO-8859-1) and abort these steps.
2. Let the url-unescaped-string be the encoded-string %-unescaped.
3. Emit the url-unescaped-string (decoded as UTF-8). (There's actually more sadness here if the url-unescaped-string 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. ...