gohugoio/hugo · error

MIME not supported

Error message

MIME %q not supported

What it means

When transform.Unmarshal receives a Resource without an explicit format option, it derives the format from the resource's MediaType suffixes via metadecoders.FormatFromStrings (unmarshal.go:81-85). If none of the media-type suffixes map to a supported format (org/json/toml/yaml/csv/xml), the resource's MediaType is reported as unsupported.

Solutions

1. Provide an explicit format option: {{ transform.Unmarshal (dict "format" "json") $res }}.
2. Ensure the resource's file extension maps to a known media type in hugo.toml ([mediaTypes]).
3. Verify the resource is the expected type before unmarshalling (e.g. check MediaType.String()).

Example fix

// before — MediaType not recognized, no explicit format
{{ transform.Unmarshal $remoteResource }}

// after — pass explicit format
{{ transform.Unmarshal (dict "format" "csv") $remoteResource }}

// Provide an explicit format instead of relying on MediaType inference:
{{ transform.Unmarshal (dict "format" "csv") $res }}
// Or ensure the resource MediaType has a suffix mapped in hugo.toml [mediaTypes].

When it happens

Trigger: Passing a Resource whose MediaType suffixes are not among the recognized set — e.g. a text/plain, application/pdf, or a custom media type without a known suffix. Encountered when no explicit format is provided.

Common situations: Fetching a remote resource with {{ resources.GetRemote }} that returns a generic content type; a custom media type configured in hugo.toml without a matching suffix; a resource whose file extension isn't registered; passing a Page or non-Resource value that has an unexpected MediaType.

Related errors

• format not supported
• failed to decode options
• invalid character
• type %T not supported
• unknown format

AI-assisted analysis of gohugoio/hugo@52c9bd7908 (2026-08-09). Data as JSON: /api/errors/7c81c4b3611da293. Report an issue: GitHub.

Appendix: source

Thrown at tpl/transform/unmarshal.go:84

		if key == "" {
			return nil, errors.New("no Key set in Resource")
		}

		if decoder != metadecoders.Default {
			key += decoder.OptionsKey()
		}

		v, err := ns.cacheUnmarshal.GetOrCreate(key, func(string) (*resources.StaleValue[any], error) {
			var f metadecoders.Format
			if decoder.Format != "" {
				f = metadecoders.FormatFromString(decoder.Format)
				if f == "" {
					return nil, fmt.Errorf("format %q not supported", decoder.Format)
				}
			} else {
				f = metadecoders.FormatFromStrings(r.MediaType().Suffixes()...)
				if f == "" {
					return nil, fmt.Errorf("MIME %q not supported", r.MediaType())
				}
			}

			reader, err := r.ReadSeekCloser()
			if err != nil {
				return nil, err
			}
			defer reader.Close()

			b, err := io.ReadAll(reader)
			if err != nil {
				return nil, err
			}

			v, err := decoder.Unmarshal(b, f)
			if err != nil {
				return nil, err
			}

View on GitHub (pinned to 52c9bd7908)