diff --git a/aegisub/FFmpegSource2/stdiostream.c b/aegisub/FFmpegSource2/stdiostream.c
index 18f6d8368..4615b94fe 100644
--- a/aegisub/FFmpegSource2/stdiostream.c
+++ b/aegisub/FFmpegSource2/stdiostream.c
@@ -98,6 +98,15 @@ int StdIoProgress(StdIoStream *st, ulonglong cur, ulonglong max) {
   return 1;
 }
 
+longlong StdIoGetFileSize(StdIoStream *st) {
+	longlong epos = 0;
+	longlong cpos = _ftelli64(st->fp);
+	_fseeki64(st->fp, 0, SEEK_END);
+	epos = _ftelli64(st->fp);
+	_fseeki64(st->fp, cpos, SEEK_SET);
+	return epos;
+}
+
 void InitStdIoStream(StdIoStream *st) {
 	memset(st,0,sizeof(st));
 	st->base.read = StdIoRead;
@@ -108,4 +117,5 @@ void InitStdIoStream(StdIoStream *st) {
 	st->base.memrealloc = StdIoRealloc;
 	st->base.memfree = StdIoFree;
 	st->base.progress = StdIoProgress;
+	st->base.getfilesize = StdIoGetFileSize;
 }
diff --git a/aegisub/FFmpegSource2/stdiostream.h b/aegisub/FFmpegSource2/stdiostream.h
index 2b8d0ae8a..b7d64b6db 100644
--- a/aegisub/FFmpegSource2/stdiostream.h
+++ b/aegisub/FFmpegSource2/stdiostream.h
@@ -36,9 +36,9 @@
  * source file
  */
 struct StdIoStream {
-  struct InputStream  base;
-  FILE		      *fp;
-  int		      error;
+  struct InputStream base;
+  FILE *fp;
+  int error;
 };
 
 typedef struct StdIoStream StdIoStream;
@@ -64,16 +64,18 @@ unsigned StdIoGetCacheSize(StdIoStream *st);
 const char *StdIoGetLastError(StdIoStream *st);
 
 /* memory allocation, this is done via stdlib */
-void  *StdIoMalloc(StdIoStream *st, size_t size);
+void *StdIoMalloc(StdIoStream *st, size_t size);
 
-void  *StdIoRealloc(StdIoStream *st, void *mem, size_t size);
+void *StdIoRealloc(StdIoStream *st, void *mem, size_t size);
 
-void  StdIoFree(StdIoStream *st, void *mem);
+void StdIoFree(StdIoStream *st, void *mem);
 
 /* progress report handler for lengthy operations
  * returns 0 to abort operation, nonzero to continue
  */
-int   StdIoProgress(StdIoStream *st, ulonglong cur, ulonglong max);
+int StdIoProgress(StdIoStream *st, ulonglong cur, ulonglong max);
+
+longlong StdIoGetFileSize(StdIoStream *st);
 
 void InitStdIoStream(StdIoStream *st);