Fix various ImageDiff/SVG bugs (#23312)

Replace #23310, Close #19733

And fix various UI problems, including regressions from #22959 #22950
and more.

## SVG Detection

The old regexp may mismatch non-SVG files. This PR adds new tests for
those cases.

## UI Changes

### Before

![image](https://user-images.githubusercontent.com/2114189/222967716-f6ad8721-f46a-4a3f-9eb0-a89e488d3436.png)

![image](https://user-images.githubusercontent.com/2114189/222967780-8af8981a-e69d-4304-9dc4-0235582fa4f4.png)

### After

![image](https://user-images.githubusercontent.com/2114189/222967575-c21c23d4-0200-4e09-aac3-57895e853000.png)

![image](https://user-images.githubusercontent.com/2114189/222967585-8b8da262-bc96-441a-9851-8d3845f2659d.png)

![image](https://user-images.githubusercontent.com/2114189/222967595-58d9bea5-6df4-41fa-bf8a-86704117959d.png)

![image](https://user-images.githubusercontent.com/2114189/222967608-38757c1a-b8bd-4ebf-b7a8-3b30edb7f303.png)

![image](https://user-images.githubusercontent.com/2114189/222967623-9849a339-6fae-4484-8fa5-939e2fdacbf5.png)

![image](https://user-images.githubusercontent.com/2114189/222967633-4383d7dd-62ba-47a3-8c10-86f7ca7757ae.png)

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

1 files changed, 15 insertions, 6 deletions

diff --git a/modules/typesniffer/typesniffer.go b/modules/typesniffer/typesniffer.go
index c9fef953c..5b215496b 100644
--- a/modules/typesniffer/typesniffer.go
+++ b/modules/typesniffer/typesniffer.go
@@ -4,6 +4,7 @@
 package typesniffer
 
 import (
+	"bytes"
 	"fmt"
 	"io"
 	"net/http"
@@ -24,8 +25,9 @@ const (
 )
 
 var (
-	svgTagRegex      = regexp.MustCompile(`(?si)\A\s*(?:(<!--.*?-->|<!DOCTYPE\s+svg([\s:]+.*?>|>))\s*)*<svg[\s>\/]`)
-	svgTagInXMLRegex = regexp.MustCompile(`(?si)\A<\?xml\b.*?\?>\s*(?:(<!--.*?-->|<!DOCTYPE\s+svg([\s:]+.*?>|>))\s*)*<svg[\s>\/]`)
+	svgComment       = regexp.MustCompile(`(?s)<!--.*?-->`)
+	svgTagRegex      = regexp.MustCompile(`(?si)\A\s*(?:(<!DOCTYPE\s+svg([\s:]+.*?>|>))\s*)*<svg\b`)
+	svgTagInXMLRegex = regexp.MustCompile(`(?si)\A<\?xml\b.*?\?>\s*(?:(<!DOCTYPE\s+svg([\s:]+.*?>|>))\s*)*<svg\b`)
 )
 
 // SniffedType contains information about a blobs type.
@@ -91,10 +93,17 @@ func DetectContentType(data []byte) SniffedType {
 		data = data[:sniffLen]
 	}
 
-	if (strings.Contains(ct, "text/plain") || strings.Contains(ct, "text/html")) && svgTagRegex.Match(data) ||
-		strings.Contains(ct, "text/xml") && svgTagInXMLRegex.Match(data) {
-		// SVG is unsupported. https://github.com/golang/go/issues/15888
-		ct = SvgMimeType
+	// SVG is unsupported by http.DetectContentType, https://github.com/golang/go/issues/15888
+
+	detectByHTML := strings.Contains(ct, "text/plain") || strings.Contains(ct, "text/html")
+	detectByXML := strings.Contains(ct, "text/xml")
+	if detectByHTML || detectByXML {
+		dataProcessed := svgComment.ReplaceAll(data, nil)
+		dataProcessed = bytes.TrimSpace(dataProcessed)
+		if detectByHTML && svgTagRegex.Match(dataProcessed) ||
+			detectByXML && svgTagInXMLRegex.Match(dataProcessed) {
+			ct = SvgMimeType
+		}
 	}
 
 	return SniffedType{ct}