Closes #29: Allow attaching/embedding from embed.FS

- Added `EmbedFromEmbedFS()` to allow embedding from embed.FS
- Added `AttachFromEmbedFS()` to allow attaching from embed.FS
- Added `fileFromEmbedFS()` as internal method for both other m
  methods to attach/embed the embed.FS file

msg.go

@@ -7,6 +7,7 @@ package mail
 import (
 	"bytes"
 	"context"
+	"embed"
 	"errors"
 	"fmt"
 	ht "html/template"
@@ -539,6 +540,19 @@ func (m *Msg) AttachTextTemplate(n string, t *tt.Template, d interface{}, o ...F
 	return nil
 }
 
+// AttachFromEmbedFS adds an attachment File from an embed.FS to the Msg
+func (m *Msg) AttachFromEmbedFS(n string, f *embed.FS, o ...FileOption) error {
+	if f == nil {
+		return fmt.Errorf("embed.FS must not be nil")
+	}
+	ef, err := fileFromEmbedFS(n, f)
+	if err != nil {
+		return err
+	}
+	m.attachments = m.appendFile(m.attachments, ef, o...)
+	return nil
+}
+
 // EmbedFile adds an embedded File to the Msg
 func (m *Msg) EmbedFile(n string, o ...FileOption) {
 	f := fileFromFS(n)
@@ -574,6 +588,19 @@ func (m *Msg) EmbedTextTemplate(n string, t *tt.Template, d interface{}, o ...Fi
 	return nil
 }
 
+// EmbedFromEmbedFS adds an embedded File from an embed.FS to the Msg
+func (m *Msg) EmbedFromEmbedFS(n string, f *embed.FS, o ...FileOption) error {
+	if f == nil {
+		return fmt.Errorf("embed.FS must not be nil")
+	}
+	ef, err := fileFromEmbedFS(n, f)
+	if err != nil {
+		return err
+	}
+	m.embeds = m.appendFile(m.embeds, ef, o...)
+	return nil
+}
+
 // Reset resets all headers, body parts and attachments/embeds of the Msg
 // It leaves already set encodings, charsets, boundaries, etc. as is
 func (m *Msg) Reset() {
@@ -767,6 +794,30 @@ func (m *Msg) addDefaultHeader() {
 	m.SetHeader(HeaderMIMEVersion, string(m.mimever))
 }
 
+// fileFromEmbedFS returns a File pointer from a given file in the provided embed.FS
+func fileFromEmbedFS(n string, f *embed.FS) (*File, error) {
+	_, err := f.Open(n)
+	if err != nil {
+		return nil, fmt.Errorf("failed to open file from embed.FS: %w", err)
+	}
+	return &File{
+		Name:   filepath.Base(n),
+		Header: make(map[string][]string),
+		Writer: func(w io.Writer) (int64, error) {
+			h, err := f.Open(n)
+			if err != nil {
+				return 0, err
+			}
+			nb, err := io.Copy(w, h)
+			if err != nil {
+				_ = h.Close()
+				return nb, fmt.Errorf("failed to copy file to io.Writer: %w", err)
+			}
+			return nb, h.Close()
+		},
+	}, nil
+}
+
 // fileFromFS returns a File pointer from a given file in the system's file system
 func fileFromFS(n string) *File {
 	_, err := os.Stat(n)

msg_test.go

@@ -7,6 +7,7 @@ package mail
 import (
 	"bufio"
 	"bytes"
+	"embed"
 	"fmt"
 	htpl "html/template"
 	"io"
@@ -18,6 +19,9 @@ import (
 	"time"
 )
 
+//go:embed README.md
+var efs embed.FS
+
 // TestNewMsg tests the NewMsg method
 func TestNewMsg(t *testing.T) {
 	m := NewMsg()
@@ -1029,6 +1033,50 @@ func TestMsg_AttachFile(t *testing.T) {
 	}
 }
 
+// TestMsg_AttachFromEmbedFS tests the Msg.AttachFromEmbedFS and the WithFilename FileOption method
+func TestMsg_AttachFromEmbedFS(t *testing.T) {
+	tests := []struct {
+		name string
+		file string
+		fn   string
+		sf   bool
+	}{
+		{"File: README.md", "README.md", "README.md", false},
+		{"File: nonexisting", "", "invalid.file", true},
+	}
+	m := NewMsg()
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if err := m.AttachFromEmbedFS(tt.file, &efs, WithFileName(tt.fn)); err != nil && !tt.sf {
+				t.Errorf("AttachFromEmbedFS() failed: %s", err)
+				return
+			}
+			if len(m.attachments) != 1 && !tt.sf {
+				t.Errorf("AttachFile() failed. Number of attachments expected: %d, got: %d", 1,
+					len(m.attachments))
+				return
+			}
+			if !tt.sf {
+				file := m.attachments[0]
+				if file == nil {
+					t.Errorf("AttachFile() failed. Attachment file pointer is nil")
+					return
+				}
+				if file.Name != tt.fn {
+					t.Errorf("AttachFile() failed. Filename of attachment expected: %s, got: %s", tt.fn,
+						file.Name)
+				}
+				buf := bytes.Buffer{}
+				if _, err := file.Writer(&buf); err != nil {
+					t.Errorf("failed to execute WriterFunc: %s", err)
+					return
+				}
+			}
+			m.Reset()
+		})
+	}
+}
+
 // TestMsg_AttachFileBrokenFunc tests WriterFunc of the Msg.AttachFile  method
 func TestMsg_AttachFileBrokenFunc(t *testing.T) {
 	m := NewMsg()
@@ -1125,6 +1173,50 @@ func TestMsg_EmbedFile(t *testing.T) {
 	}
 }
 
+// TestMsg_EmbedFromEmbedFS tests the Msg.EmbedFromEmbedFS and the WithFilename FileOption method
+func TestMsg_EmbedFromEmbedFS(t *testing.T) {
+	tests := []struct {
+		name string
+		file string
+		fn   string
+		sf   bool
+	}{
+		{"File: README.md", "README.md", "README.md", false},
+		{"File: nonexisting", "", "invalid.file", true},
+	}
+	m := NewMsg()
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if err := m.EmbedFromEmbedFS(tt.file, &efs, WithFileName(tt.fn)); err != nil && !tt.sf {
+				t.Errorf("EmbedFromEmbedFS() failed: %s", err)
+				return
+			}
+			if len(m.embeds) != 1 && !tt.sf {
+				t.Errorf("EmbedFile() failed. Number of embeds expected: %d, got: %d", 1,
+					len(m.embeds))
+				return
+			}
+			if !tt.sf {
+				file := m.embeds[0]
+				if file == nil {
+					t.Errorf("EmbedFile() failed. Embedded file pointer is nil")
+					return
+				}
+				if file.Name != tt.fn {
+					t.Errorf("EmbedFile() failed. Filename of embeds expected: %s, got: %s", tt.fn,
+						file.Name)
+				}
+				buf := bytes.Buffer{}
+				if _, err := file.Writer(&buf); err != nil {
+					t.Errorf("failed to execute WriterFunc: %s", err)
+					return
+				}
+			}
+			m.Reset()
+		})
+	}
+}
+
 // TestMsg_EmbedFileBrokenFunc tests WriterFunc of the Msg.EmbedFile  method
 func TestMsg_EmbedFileBrokenFunc(t *testing.T) {
 	m := NewMsg()