diff --git a/apps/app_mixmonitor.c b/apps/app_mixmonitor.c
index 6746c473cf..a5c27bfc8b 100644
--- a/apps/app_mixmonitor.c
+++ b/apps/app_mixmonitor.c
@@ -154,6 +154,11 @@
 			<para>This application does not automatically answer and should be preceeded by
 			an application such as Answer or Progress().</para>
 			<note><para>MixMonitor runs as an audiohook.</para></note>
+			<note><para>If a filename passed to MixMonitor ends with
+			<literal>.wav49</literal>, Asterisk will silently convert the extension to
+			<literal>.WAV</literal> for legacy reasons. <variable>MIXMONITOR_FILENAME</variable>
+			will contain the actual filename that Asterisk is writing to, not necessarily the
+			value that was passed in.</para></note>
 			<variablelist>
 				<variable name="MIXMONITOR_FILENAME">
 					<para>Will contain the filename used to record.</para>
@@ -998,17 +1003,35 @@ static int launch_monitor_thread(struct ast_channel *chan, const char *filename,
 static char *filename_parse(char *filename, char *buffer, size_t len)
 {
 	char *slash;
+	char *ext;
+
+	ast_assert(len > 0);
+
 	if (ast_strlen_zero(filename)) {
 		ast_log(LOG_WARNING, "No file name was provided for a file save option.\n");
-	} else if (filename[0] != '/') {
-		char *build;
-		build = ast_alloca(strlen(ast_config_AST_MONITOR_DIR) + strlen(filename) + 3);
+		buffer[0] = 0;
+		return buffer;
+	}
+
+	/* If we don't have an absolute path, make one */
+	if (*filename != '/') {
+		char *build = ast_alloca(strlen(ast_config_AST_MONITOR_DIR) + strlen(filename) + 3);
 		sprintf(build, "%s/%s", ast_config_AST_MONITOR_DIR, filename);
 		filename = build;
 	}
 
 	ast_copy_string(buffer, filename, len);
 
+	/* If the provided filename has a .wav49 extension, we need to convert it to .WAV to
+	   match the behavior of build_filename in main/file.c. Otherwise MIXMONITOR_FILENAME
+	   ends up referring to a file that does not/will not exist */
+	ext = strrchr(buffer, '.');
+	if (ext && !strcmp(ext, ".wav49")) {
+		/* Change to WAV - we know we have at least 6 writeable bytes where 'ext' points,
+		 * so this is safe */
+		memcpy(ext, ".WAV", sizeof(".WAV"));
+	}
+
 	if ((slash = strrchr(filename, '/'))) {
 		*slash = '\0';
 	}
diff --git a/doc/CHANGES-staging/app_mixmonitor_wav49.txt b/doc/CHANGES-staging/app_mixmonitor_wav49.txt
new file mode 100644
index 0000000000..f3218d70d2
--- /dev/null
+++ b/doc/CHANGES-staging/app_mixmonitor_wav49.txt
@@ -0,0 +1,8 @@
+Subject: app_mixmonitor
+
+If the 'filename' argument to MixMonitor() ended with '.wav49,'
+Asterisk would silently convert the extension to '.WAV' when opening
+the file for writing. This caused the MIXMONITOR_FILENAME variable to
+reference the wrong file. The MIXMONITOR_FILENAME variable will now
+reflect the name of the file that Asterisk actually used instead of
+the filename that was passed to the application.
diff --git a/main/file.c b/main/file.c
index a0616c8d02..b4266093ce 100644
--- a/main/file.c
+++ b/main/file.c
@@ -315,6 +315,8 @@ static char *build_filename(const char *filename, const char *ext)
 {
 	char *fn = NULL;
 
+	/* The wav49 -> WAV translation is duplicated in apps/app_mixmonitor.c, so
+	   if you change it here you need to change it there as well */
 	if (!strcmp(ext, "wav49"))
 		ext = "WAV";