Use .st for the default Atari ST image filenames.

src/readables/atarist.textpb

@@ -6,7 +6,7 @@ input {
 
 output {
 	image {
-		filename: "atarist.img"
+		filename: "atarist.st"
 		img {}
 	}
 }

src/writables/atarist360.textpb

@@ -1,6 +1,6 @@
 input {
 	image {
-		filename: "atarist360.img"
+		filename: "atarist360.st"
 		img {
 			tracks: 80
 			sides: 1

src/writables/atarist370.textpb

@@ -1,6 +1,6 @@
 input {
 	image {
-		filename: "atarist370.img"
+		filename: "atarist370.st"
 		img {
 			tracks: 82
 			sides: 1

src/writables/atarist400.textpb

@@ -1,6 +1,6 @@
 input {
 	image {
-		filename: "atarist400.img"
+		filename: "atarist400.st"
 		img {
 			tracks: 80
 			sides: 1

src/writables/atarist410.textpb

@@ -1,6 +1,6 @@
 input {
 	image {
-		filename: "atarist410.img"
+		filename: "atarist410.st"
 		img {
 			tracks: 82
 			sides: 1

src/writables/atarist720.textpb

@@ -1,6 +1,6 @@
 input {
 	image {
-		filename: "atarist720.img"
+		filename: "atarist720.st"
 		img {
 			tracks: 80
 			sides: 2

src/writables/atarist740.textpb

@@ -1,6 +1,6 @@
 input {
 	image {
-		filename: "atarist740.img"
+		filename: "atarist740.st"
 		img {
 			tracks: 82
 			sides: 2

src/writables/atarist800.textpb

@@ -1,6 +1,6 @@
 input {
 	image {
-		filename: "atarist800.img"
+		filename: "atarist800.st"
 		img {
 			tracks: 80
 			sides: 2

src/writables/atarist820.textpb

@@ -1,6 +1,6 @@
 input {
 	image {
-		filename: "atarist820.img"
+		filename: "atarist820.st"
 		img {
 			tracks: 82
 			sides: 2