diff --git a/Info.go b/Info.go
index 1f7adca..bc11407 100644
--- a/Info.go
+++ b/Info.go
@@ -100,6 +100,7 @@ type Fragment struct {
 	XHeadSeqNum int
 	Data        *bytes.Buffer
 	Slow        bool
+	MimeType    string
 }
 
 type seqChanInfo struct {
@@ -905,6 +906,11 @@ func (di *DownloadInfo) downloadFragment(state *fragThreadState, dataChan chan<-
 			headerSeqnum, _ = strconv.Atoi(headerSeqnumStr)
 		}
 
+		mimeType := resp.Header.Get("Content-Type")
+		if !strings.HasSuffix(mimeType, "/mp4") && !strings.HasSuffix(mimeType, "/webm") {
+			LogTrace("%s: fragment %d has unknown MIME type '%s'", state.Name, state.SeqNum, mimeType)
+		}
+
 		if state.ToFile {
 			err = os.WriteFile(fname, respData, 0644)
 			if err != nil {
@@ -936,6 +942,7 @@ func (di *DownloadInfo) downloadFragment(state *fragThreadState, dataChan chan<-
 			FileName:    fname,
 			Data:        data,
 			Slow:        isSlow,
+			MimeType:    mimeType,
 		}
 
 		return
@@ -1157,7 +1164,20 @@ func (di *DownloadInfo) DownloadStream(dataType, dataFile string, progressChan c
 			buf := make([]byte, BufferSize)
 
 			rc, _ := data.Data.Read(buf)
-			count, err := f.Write(RemoveSidx(buf[:rc]))
+
+			writeBuf := buf
+			// ffmpeg doesn't like certain atoms in concatenated MP4 files, so we remove those here
+			// If MimeType is blank, assume MP4
+			if strings.HasSuffix(data.MimeType, "/mp4") || data.MimeType == "" {
+				badAtoms := []string{"sidx"}
+				// ffmpeg 6.1 doesn't like multiple ftyp atoms, so only allow on the first fragment
+				if curFrag != startFrag {
+					badAtoms = append(badAtoms, "ftyp")
+				}
+				writeBuf = RemoveAtoms(buf[:rc], badAtoms...)
+			}
+
+			count, err := f.Write(writeBuf)
 			bytesWritten += count
 
 			if err != nil {
diff --git a/util.go b/util.go
index da8953b..3c2d0e3 100644
--- a/util.go
+++ b/util.go
@@ -19,6 +19,7 @@ import (
 	"os/signal"
 	"path/filepath"
 	"regexp"
+	"sort"
 	"strconv"
 	"strings"
 	"time"
@@ -530,19 +531,31 @@ func GetAtoms(data []byte) map[string]Atom {
 	return atoms
 }
 
-func RemoveSidx(data []byte) []byte {
+func RemoveAtoms(data []byte, atomList ...string) []byte {
 	atoms := GetAtoms(data)
-	sidx, ok := atoms["sidx"]
 
-	if !ok {
-		return data
+	var atomsToRemove []Atom
+	for _, atomName := range atomList {
+		atom, ok := atoms[atomName]
+		if !ok {
+			continue
+		}
+		atomsToRemove = append(atomsToRemove, atom)
 	}
 
-	ofs := sidx.Offset
-	rlen := sidx.Offset + sidx.Length
-	newData := append(data[:ofs], data[rlen:]...)
+	// Sort atoms by byte offset in descending order,
+	// this lets us remove them in order without affecting the next atom's offset
+	sort.Slice(atomsToRemove, func(i, j int) bool {
+		return atomsToRemove[i].Offset > atomsToRemove[j].Offset
+	})
 
-	return newData
+	for _, atom := range atomsToRemove {
+		ofs := atom.Offset
+		rlen := atom.Offset + atom.Length
+		data = append(data[:ofs], data[rlen:]...)
+	}
+
+	return data
 }
 
 func GetVideoIdFromWatchPage(data []byte) string {