Reopening mailcontent file to fix #166

Because editors like vim use backupfiles and rename them to the original
name, the file handle used can point to the wrong file. Reopening the
file should fix this.

widgets/compose.go

@@ -278,7 +278,13 @@ func (c *Composer) PrepareHeader() (*mail.Header, []string, error) {
 }
 
 func (c *Composer) WriteMessage(header *mail.Header, writer io.Writer) error {
-	c.email.Seek(0, os.SEEK_SET)
+	name := c.email.Name()
+	c.email.Close()
+	file, err := os.Open(name)
+	if err != nil {
+		return errors.Wrap(err, "FileOpen")
+	}
+	c.email = file
 	var body io.Reader
 	reader, err := mail.CreateReader(c.email)
 	if err == nil {