Fix Attach/EmbedReader and implement Attach/EmbedReadSeeker

This PR addresses #110.
`Msg.AttachReader()` would not output the attached file after consecutive writes (e.g. a write to a file and then send via Client).

This PR fixes this behaviour by first reading the io.Reader into memory and then creating a new `bytes.Reader`, which does support seeking. In the writeFunc we then seek to position 0 after a successful `io.Copy`. This is probably not the most memory efficient way of handling this, but otherwise we'll have to break the `io.Reader` interface.

Additionally, a new way of attaching/embedding files has been added: `Msg.AttachReadSeeker()` and `Msg.EmbedReadSeeker()` which take a ´io.ReadSeeker` as argument instead. These two methods will skip the reading into memory and make use of the `Seek` method of the corresponding interface instead.

msg.go

@@ -719,6 +719,12 @@ func (m *Msg) AttachReader(n string, r io.Reader, o ...FileOption) {
 	m.attachments = m.appendFile(m.attachments, f, o...)
 }
 
+// AttachReadSeeker adds an attachment File via io.ReadSeeker to the Msg
+func (m *Msg) AttachReadSeeker(n string, r io.ReadSeeker, o ...FileOption) {
+	f := fileFromReadSeeker(n, r)
+	m.attachments = m.appendFile(m.attachments, f, o...)
+}
+
 // AttachHTMLTemplate adds the output of a html/template.Template pointer as File attachment to the Msg
 func (m *Msg) AttachHTMLTemplate(n string, t *ht.Template, d interface{}, o ...FileOption) error {
 	f, err := fileFromHTMLTemplate(n, t, d)
@@ -767,6 +773,12 @@ func (m *Msg) EmbedReader(n string, r io.Reader, o ...FileOption) {
 	m.embeds = m.appendFile(m.embeds, f, o...)
 }
 
+// EmbedReadSeeker adds an embedded File from an io.ReadSeeker to the Msg
+func (m *Msg) EmbedReadSeeker(n string, r io.ReadSeeker, o ...FileOption) {
+	f := fileFromReadSeeker(n, r)
+	m.embeds = m.appendFile(m.embeds, f, o...)
+}
+
 // EmbedHTMLTemplate adds the output of a html/template.Template pointer as embedded File to the Msg
 func (m *Msg) EmbedHTMLTemplate(n string, t *ht.Template, d interface{}, o ...FileOption) error {
 	f, err := fileFromHTMLTemplate(n, t, d)
@@ -1114,15 +1126,37 @@ func fileFromFS(n string) *File {
 
 // fileFromReader returns a File pointer from a given io.Reader
 func fileFromReader(n string, r io.Reader) *File {
+	d, err := io.ReadAll(r)
+	if err != nil {
+		return &File{}
+	}
+	br := bytes.NewReader(d)
 	return &File{
-		Name:   filepath.Base(n),
+		Name:   n,
 		Header: make(map[string][]string),
 		Writer: func(w io.Writer) (int64, error) {
-			nb, err := io.Copy(w, r)
+			rb, cerr := io.Copy(w, br)
-			if err != nil {
+			if cerr != nil {
-				return nb, err
+				return rb, cerr
 			}
-			return nb, nil
+			_, cerr = br.Seek(0, io.SeekStart)
+			return rb, cerr
+		},
+	}
+}
+
+// fileFromReadSeeker returns a File pointer from a given io.ReadSeeker
+func fileFromReadSeeker(n string, r io.ReadSeeker) *File {
+	return &File{
+		Name:   n,
+		Header: make(map[string][]string),
+		Writer: func(w io.Writer) (int64, error) {
+			rb, err := io.Copy(w, r)
+			if err != nil {
+				return rb, err
+			}
+			_, err = r.Seek(0, io.SeekStart)
+			return rb, err
 		},
 	}
 }