Try a bit harder to provide a clear error message.

When an unexpected file format is provided for a chart archive, spend
some extra effort to provide the user with some meaningful feedback.

This is intended to help users who mistakenly invoke `helm template`
like this:

    helm template values.yaml charts/mychart

This is a user error, and now we can report that the argument
`values.yaml` was expected to be a chart.

Fixes #5296

Signed-off-by: Jordan Sissel <jls@semicomplete.com>

pkg/chartutil/load.go

@@ -24,6 +24,7 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"net/http"
 	"os"
 	"path"
 	"path/filepath"
@@ -255,7 +256,45 @@ func LoadFile(name string) (*chart.Chart, error) {
 	}
 	defer raw.Close()
 
-	return LoadArchive(raw)
+	err = ensureArchive(name, raw)
+	if err != nil {
+		return nil, err
+	}
+
+	c, err := LoadArchive(raw)
+	if err != nil {
+		if err == gzip.ErrHeader {
+			return nil, fmt.Errorf("file '%s' does not appear to be a valid chart file (details: %s)", name, err)
+		}
+	}
+	return c, err
+}
+
+// ensureArchive's job is to return an informative error if the file does not appear to be a gzipped archive.
+//
+// Sometimes users will provide a values.yaml for an argument where a chart is expected. One common occurrence
+// of this is invoking `helm template values.yaml mychart` which would otherwise produce a confusing error
+// if we didn't check for this.
+func ensureArchive(name string, raw *os.File) error {
+	defer raw.Seek(0, 0) // reset read offset to allow archive loading to proceed.
+
+	// Check the file format to give us a chance to provide the user with more actionable feedback.
+	buffer := make([]byte, 512)
+	_, err := raw.Read(buffer)
+	if err != nil && err != io.EOF {
+		return fmt.Errorf("file '%s' cannot be read: %s", name, err)
+	}
+	if contentType := http.DetectContentType(buffer); contentType != "application/x-gzip" {
+		// TODO: Is there a way to reliably test if a file content is YAML? ghodss/yaml accepts a wide
+		//       variety of content (Makefile, .zshrc) as valid YAML without errors.
+
+		// Wrong content type. Let's check if it's yaml and give an extra hint?
+		if strings.HasSuffix(name, ".yml") || strings.HasSuffix(name, ".yaml") {
+			return fmt.Errorf("file '%s' seems to be a YAML file, but I expected a gzipped archive", name)
+		}
+		return fmt.Errorf("file '%s' does not appear to be a gzipped archive; got '%s'", name, contentType)
+	}
+	return nil
 }
 
 // LoadDir loads from a directory.