mime: ParseMediaType should not return an error if mediatype is empty

[aldenquimby]

Go version

go version 1.23.0

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='arm64'
GOBIN=''
GOCACHE='/Users/alden/Library/Caches/go-build'
GOENV='/Users/alden/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/alden/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/alden/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/opt/homebrew/Cellar/go/1.23.1/libexec'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='local'
GOTOOLDIR='/opt/homebrew/Cellar/go/1.23.1/libexec/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.23.1'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/alden/Library/Application Support/go/telemetry'
GCCGO='gccgo'
GOARM64='v8.0'
AR='ar'
CC='cc'
CXX='c++'
CGO_ENABLED='1'
GOMOD='/dev/null'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/_h/_w_mjtt91s1dws8zlxnnjpzm0000gn/T/go-build1772735711=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

I have a file server that returns Content-Disposition headers like this: ;filename*=UTF-8''hello-world.pdf

Example: https://go.dev/play/p/rxRNVZGDOQu

What did you see happen?

mime.ParseMediaType fails with the error "mime: no media type" (from here in code)

What did you expect to see?

• mime.ParseMediaType succeeds, returning mediatype "" and params["filename"] hello-world.pdf
• Note that all modern browsers and email clients parse a blank mediatype correctly
• rfc6266 and rfc2183 both say Unknown or unhandled disposition types SHOULD be handled by recipients the same way as "attachment", and presumably an empty string disposition type fits into that

[gabyhelp]

Related Issues and Documentation

• mime: handling invalid mime media parameters #19498 (closed)
• mime: ParseMediaType reports an error on value that is not quoted but contains a space #43721 (closed)
• mime: FormatMediaType does not quote filename #46200
• mime: ParseMediaType drops value with trailing semicolon #46323 (closed)
• mime: ParseMediaType can parse something FormatMediaType can't reformat #56614
• mime: ParseMediaType parses invalid media types #11289 (closed)
• mime: FormatMediaType doesn't allow to create Content-Disposition #9624 (closed)
• mime: Loosen the ParseMediaType error checking #13448 (closed)
• net/mail: ReadMessage returns EOF error when the body is not present #47886 (closed)
• net/http: ParseForm does not treat missing Content-Type as application/octet-stream #6616 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[cagedmantis]

cc @neild

[ianlancetaylor]

What should mime.ParseMediaType("") return?

[aldenquimby]

What should mime.ParseMediaType("") return?

@ianlancetaylor it's hard for me to tell from the spec, but I believe that would be invalid and should throw. The chromium parsing code starts here, for reference: https://github.com/chromium/chromium/blob/main/net/http/http_content_disposition.cc#L397

They appear to treat the disposition type and filename parsing separately. And the only completely invalid header is when there is no disposition type or filename

[Zxilly]

If we follow the logic in https://github.com/chromium/chromium/blob/04b3bd7342eb7ca8f114c7a5c5e466d802f9522b/net/http/http_content_disposition.cc#L350-L377, maybe we should set it to inline if we met an empty media type?