diff --git a/server/media/fs/filesys.go b/server/media/fs/filesys.go
index 8a56e01..fa26e11 100644
--- a/server/media/fs/filesys.go
+++ b/server/media/fs/filesys.go
@@ -9,7 +9,6 @@ import (
 	"errors"
 	"hash/fnv"
 	"io"
-	"mime"
 	"net/http"
 	"net/url"
 	"os"
@@ -141,10 +140,7 @@ func (fh *fshandler) Upload(fdef *types.FileDef, file io.Reader) (string, int64,
 	}
 
 	fname := fdef.Id
-	ext, _ := mime.ExtensionsByType(fdef.MimeType)
-	if len(ext) > 0 {
-		fname += ext[0]
-	}
+	fname += media.ExtensionByMimeType(fdef.MimeType)
 
 	fdef.Location = location
 	// Use file path to create ETag. File paths are unique so will be the ETag.
diff --git a/server/media/media.go b/server/media/media.go
index ab66baf..906e01a 100644
--- a/server/media/media.go
+++ b/server/media/media.go
@@ -4,6 +4,7 @@ package media
 import (
 	"errors"
 	"io"
+	"mime"
 	"net/http"
 	"net/url"
 	"path"
@@ -68,6 +69,37 @@ type AllowedOrigin struct {
 
 var fileNamePattern = regexp.MustCompile(`^[-_A-Za-z0-9]+`)
 
+// ExtensionByMimeType returns a stable URL extension for uploaded media.
+func ExtensionByMimeType(mimeType string) string {
+	switch strings.ToLower(strings.TrimSpace(strings.Split(mimeType, ";")[0])) {
+	case "audio/x-m4a", "audio/m4a", "audio/mp4", "audio/mp4a-latm":
+		return ".m4a"
+	case "audio/aac", "audio/aacp":
+		return ".aac"
+	case "audio/mpeg", "audio/mp3":
+		return ".mp3"
+	case "audio/ogg":
+		return ".ogg"
+	case "audio/wav", "audio/x-wav", "audio/wave":
+		return ".wav"
+	case "audio/webm":
+		return ".webm"
+	case "image/pjpeg", "image/jpg":
+		return ".jpg"
+	case "image/jpeg":
+		return ".jpg"
+	case "image/jfif":
+		return ".jfif"
+	case "video/mp4":
+		return ".mp4"
+	}
+	exts, _ := mime.ExtensionsByType(mimeType)
+	if len(exts) > 0 {
+		return exts[0]
+	}
+	return ""
+}
+
 // GetIdFromUrl is a helper method for extracting file ID from a URL.
 func GetIdFromUrl(url, serveUrl string) types.Uid {
 	dir, fname := path.Split(path.Clean(url))
diff --git a/server/media/media_test.go b/server/media/media_test.go
index a5f85f4..8dafe02 100644
--- a/server/media/media_test.go
+++ b/server/media/media_test.go
@@ -219,3 +219,21 @@ func TestMatchCORSOrigin(t *testing.T) {
 func containsSubstring(str, substr string) bool {
 	return strings.Contains(strings.ToLower(str), strings.ToLower(substr))
 }
+
+func TestExtensionByMimeType(t *testing.T) {
+	cases := map[string]string{
+		"audio/x-m4a":              ".m4a",
+		"audio/x-m4a; charset=utf": ".m4a",
+		"audio/mp4":                ".m4a",
+		"audio/aac":                ".aac",
+		"audio/mpeg":               ".mp3",
+		"image/jpeg":               ".jpg",
+		"image/jfif":               ".jfif",
+		"video/mp4":                ".mp4",
+	}
+	for mimeType, want := range cases {
+		if got := ExtensionByMimeType(mimeType); got != want {
+			t.Fatalf("ExtensionByMimeType(%q)=%q, want %q", mimeType, got, want)
+		}
+	}
+}
diff --git a/server/media/oss/oss.go b/server/media/oss/oss.go
index 4aaba39..d5b8629 100644
--- a/server/media/oss/oss.go
+++ b/server/media/oss/oss.go
@@ -7,7 +7,6 @@ import (
 	"errors"
 	"hash/fnv"
 	"io"
-	"mime"
 	"net/http"
 	"net/url"
 	"path"
@@ -247,10 +246,7 @@ func (oh *ossHandler) Upload(fdef *types.FileDef, file io.Reader) (string, int64
 	}
 
 	fname := fdef.Id
-	ext, _ := mime.ExtensionsByType(fdef.MimeType)
-	if len(ext) > 0 {
-		fname += ext[0]
-	}
+	fname += media.ExtensionByMimeType(fdef.MimeType)
 
 	fdef.Location = key
 	fdef.ETag = etagFromLocation(key)
diff --git a/server/media/s3/s3.go b/server/media/s3/s3.go
index 2bc84b4..f75acf4 100644
--- a/server/media/s3/s3.go
+++ b/server/media/s3/s3.go
@@ -6,7 +6,6 @@ import (
 	"encoding/json"
 	"errors"
 	"io"
-	"mime"
 	"net/http"
 	"net/url"
 	"path"
@@ -280,22 +279,23 @@ func (ah *awshandler) Upload(fdef *types.FileDef, file io.Reader) (string, int64
 	}
 
 	rc := readerCounter{reader: file}
-	result, err := tmClient.UploadObject(context.Background(), &transfermanager.UploadObjectInput{
+	input := &transfermanager.UploadObjectInput{
 		CacheControl: aws.String(ah.conf.CacheControl),
 		Bucket:       aws.String(ah.conf.BucketName),
 		Key:          aws.String(key),
 		Body:         &rc,
-	})
+	}
+	if fdef.MimeType != "" {
+		input.ContentType = aws.String(fdef.MimeType)
+	}
+	result, err := tmClient.UploadObject(context.Background(), input)
 
 	if err != nil {
 		return "", 0, err
 	}
 
 	fname := fdef.Id
-	ext, _ := mime.ExtensionsByType(fdef.MimeType)
-	if len(ext) > 0 {
-		fname += ext[0]
-	}
+	fname += media.ExtensionByMimeType(fdef.MimeType)
 
 	fdef.Location = key
 	if result.ETag != nil {