lib/rest: add opts.MultipartContentType to explicitly set Content-Type of attachements

Before this the standard library set it to application/octet-stream for some reason

backend/imagekit/client/upload.go

@@ -72,7 +72,7 @@ func (ik *ImageKit) Upload(ctx context.Context, file io.Reader, param UploadPara
 
 	response := &UploadResult{}
 
-	formReader, contentType, _, err := rest.MultipartUpload(ctx, file, formParams, "file", param.FileName)
+	formReader, contentType, _, err := rest.MultipartUpload(ctx, file, formParams, "file", param.FileName, "application/octet-stream")
 
 	if err != nil {
 		return nil, nil, fmt.Errorf("failed to make multipart upload: %w", err)

backend/pcloud/pcloud.go

@@ -1459,7 +1459,7 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 	// opts.Body=0), so upload it as a multipart form POST with
 	// Content-Length set.
 	if size == 0 {
-		formReader, contentType, overhead, err := rest.MultipartUpload(ctx, in, opts.Parameters, "content", leaf)
+		formReader, contentType, overhead, err := rest.MultipartUpload(ctx, in, opts.Parameters, "content", leaf, opts.ContentType)
 		if err != nil {
 			return fmt.Errorf("failed to make multipart upload for 0 length file: %w", err)
 		}

backend/pikpak/pikpak.go

@@ -1384,7 +1384,7 @@ func (f *Fs) uploadByForm(ctx context.Context, in io.Reader, name string, size i
 	for i := range iVal.NumField() {
 		params.Set(iTyp.Field(i).Tag.Get("json"), iVal.Field(i).String())
 	}
-	formReader, contentType, overhead, err := rest.MultipartUpload(ctx, in, params, "file", name)
+	formReader, contentType, overhead, err := rest.MultipartUpload(ctx, in, params, "file", name, "application/octet-stream")
 	if err != nil {
 		return fmt.Errorf("failed to make multipart upload: %w", err)
 	}

backend/seafile/webapi.go

@@ -688,7 +688,7 @@ func (f *Fs) upload(ctx context.Context, in io.Reader, uploadLink, filePath stri
 		"need_idx_progress": {"true"},
 		"replace":           {"1"},
 	}
-	formReader, contentType, _, err := rest.MultipartUpload(ctx, in, parameters, "file", f.opt.Enc.FromStandardName(filename))
+	formReader, contentType, _, err := rest.MultipartUpload(ctx, in, parameters, "file", f.opt.Enc.FromStandardName(filename), "application/octet-stream")
 	if err != nil {
 		return nil, fmt.Errorf("failed to make multipart upload: %w", err)
 	}

backend/zoho/zoho.go

@@ -817,7 +817,7 @@ func (f *Fs) upload(ctx context.Context, name string, parent string, size int64,
 	params.Set("filename", url.QueryEscape(name))
 	params.Set("parent_id", parent)
 	params.Set("override-name-exist", strconv.FormatBool(true))
-	formReader, contentType, overhead, err := rest.MultipartUpload(ctx, in, nil, "content", name)
+	formReader, contentType, overhead, err := rest.MultipartUpload(ctx, in, nil, "content", name, "application/octet-stream")
 	if err != nil {
 		return nil, fmt.Errorf("failed to make multipart upload: %w", err)
 	}

fs/operations/rc_test.go

@@ -561,7 +561,7 @@ func TestUploadFile(t *testing.T) {
 		assert.NoError(t, currentFile.Close())
 	}()
 
-	formReader, contentType, _, err := rest.MultipartUpload(ctx, currentFile, url.Values{}, "file", testFileName)
+	formReader, contentType, _, err := rest.MultipartUpload(ctx, currentFile, url.Values{}, "file", testFileName, "application/octet-stream")
 	require.NoError(t, err)
 
 	httpReq := httptest.NewRequest("POST", "/", formReader)
@@ -587,7 +587,7 @@ func TestUploadFile(t *testing.T) {
 		assert.NoError(t, currentFile2.Close())
 	}()
 
-	formReader, contentType, _, err = rest.MultipartUpload(ctx, currentFile2, url.Values{}, "file", testFileName)
+	formReader, contentType, _, err = rest.MultipartUpload(ctx, currentFile2, url.Values{}, "file", testFileName, "application/octet-stream")
 	require.NoError(t, err)
 
 	httpReq = httptest.NewRequest("POST", "/", formReader)

lib/rest/rest.go

@@ -14,7 +14,9 @@ import (
 	"maps"
 	"mime/multipart"
 	"net/http"
+	"net/textproto"
 	"net/url"
+	"strings"
 	"sync"
 
 	"github.com/rclone/rclone/fs"
@@ -145,6 +147,7 @@ type Opts struct {
 	MultipartMetadataName string       // ..this is used for the name of the metadata form part if set
 	MultipartContentName  string       // ..name of the parameter which is the attached file
 	MultipartFileName     string       // ..name of the file for the attached file
+	MultipartContentType  string       // ..content type of the attached file
 	Parameters            url.Values   // any parameters for the final URL
 	TransferEncoding      []string     // transfer encoding, set to "identity" to disable chunked encoding
 	Trailer               *http.Header // set the request trailer
@@ -371,6 +374,32 @@ func (api *Client) Call(ctx context.Context, opts *Opts) (resp *http.Response, e
 	return resp, nil
 }
 
+var quoteEscaper = strings.NewReplacer("\\", "\\\\", `"`, "\\\"")
+
+func escapeQuotes(s string) string {
+	return quoteEscaper.Replace(s)
+}
+
+// multipartFileContentDisposition returns the value of a Content-Disposition header
+// with the provided field name and file name.
+func multipartFileContentDisposition(fieldname, filename string) string {
+	return fmt.Sprintf(`form-data; name="%s"; filename="%s"`,
+		escapeQuotes(fieldname), escapeQuotes(filename))
+}
+
+// CreateFormFile is a convenience wrapper around [Writer.CreatePart]. It creates
+// a new form-data header with the provided field name and file name.
+func CreateFormFile(w *multipart.Writer, fieldname, filename, contentType string) (io.Writer, error) {
+	h := make(textproto.MIMEHeader)
+	// FIXME when go1.24 is no longer supported, change to
+	// multipart.FileContentDisposition and remove definition above
+	h.Set("Content-Disposition", multipartFileContentDisposition(fieldname, filename))
+	if contentType != "" {
+		h.Set("Content-Type", contentType)
+	}
+	return w.CreatePart(h)
+}
+
 // MultipartUpload creates an io.Reader which produces an encoded a
 // multipart form upload from the params passed in and the  passed in
 //
@@ -382,10 +411,10 @@ func (api *Client) Call(ctx context.Context, opts *Opts) (resp *http.Response, e
 // the int64 returned is the overhead in addition to the file contents, in case Content-Length is required
 //
 // NB This doesn't allow setting the content type of the attachment
-func MultipartUpload(ctx context.Context, in io.Reader, params url.Values, contentName, fileName string) (io.ReadCloser, string, int64, error) {
+func MultipartUpload(ctx context.Context, in io.Reader, params url.Values, contentName, fileName string, contentType string) (io.ReadCloser, string, int64, error) {
 	bodyReader, bodyWriter := io.Pipe()
 	writer := multipart.NewWriter(bodyWriter)
-	contentType := writer.FormDataContentType()
+	formContentType := writer.FormDataContentType()
 
 	// Create a Multipart Writer as base for calculating the Content-Length
 	buf := &bytes.Buffer{}
@@ -404,7 +433,7 @@ func MultipartUpload(ctx context.Context, in io.Reader, params url.Values, conte
 		}
 	}
 	if in != nil {
-		_, err = dummyMultipartWriter.CreateFormFile(contentName, fileName)
+		_, err = CreateFormFile(dummyMultipartWriter, contentName, fileName, contentType)
 		if err != nil {
 			return nil, "", 0, err
 		}
@@ -445,7 +474,7 @@ func MultipartUpload(ctx context.Context, in io.Reader, params url.Values, conte
 		}
 
 		if in != nil {
-			part, err := writer.CreateFormFile(contentName, fileName)
+			part, err := CreateFormFile(writer, contentName, fileName, contentType)
 			if err != nil {
 				_ = bodyWriter.CloseWithError(fmt.Errorf("failed to create form file: %w", err))
 				return
@@ -467,7 +496,7 @@ func MultipartUpload(ctx context.Context, in io.Reader, params url.Values, conte
 		_ = bodyWriter.Close()
 	}()
 
-	return bodyReader, contentType, multipartLength, nil
+	return bodyReader, formContentType, multipartLength, nil
 }
 
 // CallJSON runs Call and decodes the body as a JSON object into response (if not nil)
@@ -539,7 +568,7 @@ func (api *Client) callCodec(ctx context.Context, opts *Opts, request any, respo
 		opts = opts.Copy()
 
 		var overhead int64
-		opts.Body, opts.ContentType, overhead, err = MultipartUpload(ctx, opts.Body, params, opts.MultipartContentName, opts.MultipartFileName)
+		opts.Body, opts.ContentType, overhead, err = MultipartUpload(ctx, opts.Body, params, opts.MultipartContentName, opts.MultipartFileName, opts.MultipartContentType)
 		if err != nil {
 			return nil, err
 		}