diff --git a/block.go b/block.go
index 74e89a6..744272f 100644
--- a/block.go
+++ b/block.go
@@ -1,8 +1,11 @@
 package proton
 
 import (
+	"bytes"
 	"context"
 	"io"
+	"mime/multipart"
+	"net/http"
 
 	"github.com/go-resty/resty/v2"
 )
@@ -33,10 +36,33 @@ func (c *Client) RequestBlockUpload(ctx context.Context, req BlockUploadReq) ([]
 }
 
 func (c *Client) UploadBlock(ctx context.Context, bareURL, token string, block io.Reader) error {
+	var body bytes.Buffer
+	writer := multipart.NewWriter(&body)
+	part, err := writer.CreateFormFile("Block", "blob")
+	if err != nil {
+		return err
+	}
+	if _, err := io.Copy(part, block); err != nil {
+		return err
+	}
+	if err := writer.Close(); err != nil {
+		return err
+	}
+	contentType := writer.FormDataContentType()
+	payload := body.Bytes()
+
 	return c.do(ctx, func(r *resty.Request) (*resty.Response, error) {
 		return r.
 			SetHeader("pm-storage-token", token).
-			SetMultipartField("Block", "blob", "application/octet-stream", block).
+			SetHeader("Content-Type", contentType).
+			SetBody(payload).
+			AddRetryCondition(func(res *resty.Response, _ error) bool {
+				if res == nil {
+					return false
+				}
+				return res.StatusCode() >= http.StatusInternalServerError &&
+					res.StatusCode() < 600
+			}).
 			Post(bareURL)
 	})
 }
diff --git a/block_test.go b/block_test.go
new file mode 100644
index 0000000..11941d9
--- /dev/null
+++ b/block_test.go
@@ -0,0 +1,130 @@
+package proton_test
+
+import (
+	"bytes"
+	"context"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/rclone/go-proton-api"
+	"github.com/stretchr/testify/require"
+)
+
+func TestUploadBlockReplaysMultipartBodyAfterConnectionDrop(t *testing.T) {
+	payload := []byte("encrypted Proton block payload")
+
+	var (
+		mu       sync.Mutex
+		attempts [][]byte
+	)
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		var received []byte
+		file, _, err := r.FormFile("Block")
+		if err == nil {
+			received, err = io.ReadAll(file)
+			require.NoError(t, err)
+			require.NoError(t, file.Close())
+		}
+
+		mu.Lock()
+		attempts = append(attempts, received)
+		attempt := len(attempts)
+		mu.Unlock()
+
+		if attempt == 1 {
+			hijacker, ok := w.(http.Hijacker)
+			require.True(t, ok)
+			conn, _, err := hijacker.Hijack()
+			require.NoError(t, err)
+			require.NoError(t, conn.Close())
+			return
+		}
+
+		w.Header().Set("Date", time.Now().UTC().Format(http.TimeFormat))
+		w.Header().Set("Content-Type", "application/json")
+		_, err = io.WriteString(w, `{"Code":1000}`)
+		require.NoError(t, err)
+	}))
+	defer server.Close()
+
+	manager := proton.New(
+		proton.WithHostURL(server.URL),
+		proton.WithRetryCount(1),
+	)
+	defer manager.Close()
+	client := manager.NewClient("", "", "")
+	defer client.Close()
+
+	err := client.UploadBlock(
+		context.Background(),
+		server.URL+"/storage/blocks",
+		"test-token",
+		bytes.NewReader(payload),
+	)
+	require.NoError(t, err)
+
+	mu.Lock()
+	defer mu.Unlock()
+	require.Len(t, attempts, 2)
+	require.Equal(t, payload, attempts[0])
+	require.Equal(t, payload, attempts[1])
+}
+
+func TestUploadBlockRetriesBadGatewayWithSameBody(t *testing.T) {
+	payload := []byte("encrypted block survives 502 retry")
+
+	var (
+		mu       sync.Mutex
+		attempts [][]byte
+	)
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		file, _, err := r.FormFile("Block")
+		require.NoError(t, err)
+		received, err := io.ReadAll(file)
+		require.NoError(t, err)
+		require.NoError(t, file.Close())
+
+		mu.Lock()
+		attempts = append(attempts, received)
+		attempt := len(attempts)
+		mu.Unlock()
+
+		w.Header().Set("Date", time.Now().UTC().Format(http.TimeFormat))
+		w.Header().Set("Content-Type", "application/json")
+		if attempt == 1 {
+			w.WriteHeader(http.StatusBadGateway)
+			_, err = io.WriteString(w, `{"Code":0,"Error":"simulated bad gateway"}`)
+			require.NoError(t, err)
+			return
+		}
+		_, err = io.WriteString(w, `{"Code":1000}`)
+		require.NoError(t, err)
+	}))
+	defer server.Close()
+
+	manager := proton.New(
+		proton.WithHostURL(server.URL),
+		proton.WithRetryCount(1),
+	)
+	defer manager.Close()
+	client := manager.NewClient("", "", "")
+	defer client.Close()
+
+	err := client.UploadBlock(
+		context.Background(),
+		server.URL+"/storage/blocks",
+		"test-token",
+		bytes.NewReader(payload),
+	)
+	require.NoError(t, err)
+
+	mu.Lock()
+	defer mu.Unlock()
+	require.Len(t, attempts, 2)
+	require.Equal(t, payload, attempts[0])
+	require.Equal(t, payload, attempts[1])
+}