Use extensions for querying the mime type of attachments

This gets rid of the issue that lots of things are detected as zip files, even
though they are a more specialized format (say office files)

widgets/compose.go

@@ -460,8 +460,6 @@ func writeMultipartBody(body io.Reader, w *mail.Writer) error {
 
 // write the attachment specified by path to the message
 func writeAttachment(path string, writer *mail.Writer) error {
-	filename := filepath.Base(path)
-
 	f, err := os.Open(path)
 	if err != nil {
 		return errors.Wrap(err, "os.Open")
@@ -470,20 +468,30 @@ func writeAttachment(path string, writer *mail.Writer) error {
 
 	reader := bufio.NewReader(f)
 
-	// determine the MIME type
-	// http.DetectContentType only cares about the first 512 bytes
-	head, err := reader.Peek(512)
-	if err != nil && err != io.EOF {
-		return errors.Wrap(err, "Peek")
+	// if we have an extension, prefer that instead of trying to sniff the header.
+	// That's generally more accurate than sniffing as lots of things are zip files
+	// under the hood, e.g. most office file types
+	ext := filepath.Ext(path)
+	var mimeString string
+	if mimeString = mime.TypeByExtension(ext); mimeString != "" {
+		// found it in the DB
+	} else {
+		// Sniff the mime type instead
+		// http.DetectContentType only cares about the first 512 bytes
+		head, err := reader.Peek(512)
+		if err != nil && err != io.EOF {
+			return errors.Wrap(err, "Peek")
+		}
+		mimeString = http.DetectContentType(head)
 	}
 
-	mimeString := http.DetectContentType(head)
 	// mimeString can contain type and params (like text encoding),
 	// so we need to break them apart before passing them to the headers
 	mimeType, params, err := mime.ParseMediaType(mimeString)
 	if err != nil {
 		return errors.Wrap(err, "ParseMediaType")
 	}
+	filename := filepath.Base(path)
 	params["name"] = filename
 
 	// set header fields