diff --git a/contrib/errorprone/readme.adoc b/contrib/errorprone/readme.adoc
index af67982b5d3..159b045051a 100644
--- a/contrib/errorprone/readme.adoc
+++ b/contrib/errorprone/readme.adoc
@@ -5,7 +5,7 @@ https://errorprone.info/index[Error Prone] augments the Java compiler's type che
 
 You just need to mix the `ErrorProneModule` into your `JavaModule` and it will automatically run with every compilation.
 
-.`build.mill.sc`: Enable `ErrorProne` in a module
+.`build.mill.scala`: Enable `ErrorProne` in a module
 [source,scala]
 ----
 package build
diff --git a/example/depth/large/13-helper-files-mill-sc/build.mill.sc b/example/depth/large/13-helper-files-mill-scala/build.mill.scala
similarity index 90%
rename from example/depth/large/13-helper-files-mill-sc/build.mill.sc
rename to example/depth/large/13-helper-files-mill-scala/build.mill.scala
index 7a757d7b0ba..0a0b47444fa 100644
--- a/example/depth/large/13-helper-files-mill-sc/build.mill.sc
+++ b/example/depth/large/13-helper-files-mill-scala/build.mill.scala
@@ -8,9 +8,9 @@ object `package` extends RootModule with MyModule{
     "MY_PROJECT_VERSION" -> versions.myProjectVersion,
   )
 }
-///** See Also: util.mill.sc */
-///** See Also: foo/package.mill.sc */
-///** See Also: foo/versions.mill.sc */
+///** See Also: util.mill.scala */
+///** See Also: foo/package.mill.scala */
+///** See Also: foo/versions.mill.scala */
 
 
 // Apart from having `package` files in subfolders to define modules, Mill
diff --git a/example/depth/large/13-helper-files-mill-sc/foo/package.mill.sc b/example/depth/large/13-helper-files-mill-scala/foo/package.mill.scala
similarity index 100%
rename from example/depth/large/13-helper-files-mill-sc/foo/package.mill.sc
rename to example/depth/large/13-helper-files-mill-scala/foo/package.mill.scala
diff --git a/example/depth/large/13-helper-files-mill-sc/foo/src/Foo.scala b/example/depth/large/13-helper-files-mill-scala/foo/src/Foo.scala
similarity index 100%
rename from example/depth/large/13-helper-files-mill-sc/foo/src/Foo.scala
rename to example/depth/large/13-helper-files-mill-scala/foo/src/Foo.scala
diff --git a/example/depth/large/13-helper-files-mill-sc/foo/versions.mill.sc b/example/depth/large/13-helper-files-mill-scala/foo/versions.mill.scala
similarity index 100%
rename from example/depth/large/13-helper-files-mill-sc/foo/versions.mill.sc
rename to example/depth/large/13-helper-files-mill-scala/foo/versions.mill.scala
diff --git a/example/depth/large/13-helper-files-mill-sc/src/Main.scala b/example/depth/large/13-helper-files-mill-scala/src/Main.scala
similarity index 100%
rename from example/depth/large/13-helper-files-mill-sc/src/Main.scala
rename to example/depth/large/13-helper-files-mill-scala/src/Main.scala
diff --git a/example/depth/large/13-helper-files-mill-sc/util.mill.sc b/example/depth/large/13-helper-files-mill-scala/util.mill.scala
similarity index 100%
rename from example/depth/large/13-helper-files-mill-sc/util.mill.sc
rename to example/depth/large/13-helper-files-mill-scala/util.mill.scala
diff --git a/main/client/src/mill/main/client/CodeGenConstants.java b/main/client/src/mill/main/client/CodeGenConstants.java
index 71136e2d1dd..5a5f1b55733 100644
--- a/main/client/src/mill/main/client/CodeGenConstants.java
+++ b/main/client/src/mill/main/client/CodeGenConstants.java
@@ -21,17 +21,17 @@ public class CodeGenConstants {
     /**
      * The name of the root build file
      */
-    final public static String[] rootBuildFileNames = {"build.mill", "build.mill.sc", "build.sc"};
+    final public static String[] rootBuildFileNames = {"build.mill", "build.mill.scala", "build.sc"};
 
     /**
      * The name of any sub-folder build files
      */
-    final public static String[] nestedBuildFileNames = {"package.mill", "package.mill.sc", "package.sc"};
+    final public static String[] nestedBuildFileNames = {"package.mill", "package.mill.scala", "package.sc"};
 
     /**
      * The extensions used by build files
      */
-    final public static String[] buildFileExtensions = {"mill", "mill.sc", "sc"};
+    final public static String[] buildFileExtensions = {"mill", "mill.scala", "sc"};
 
     /**
      * The user-facing name for the root of the module tree.
diff --git a/mill-build/src/ExampleParser.scala b/mill-build/src/ExampleParser.scala
index 39988ee2a86..de7b82c7cd8 100644
--- a/mill-build/src/ExampleParser.scala
+++ b/mill-build/src/ExampleParser.scala
@@ -6,9 +6,15 @@ object ExampleParser {
     val states = collection.mutable.Buffer("scala")
     val chunks = collection.mutable.Buffer(collection.mutable.Buffer.empty[String])
 
-    val rootBuildFileNames = Seq("build.sc", "build.mill", "build.mill.sc")
-    val buildFile = rootBuildFileNames.map(testRepoRoot / _).find(os.exists)
-    for (line <- os.read.lines(buildFile.get)) {
+    val rootBuildFileNames = Seq("build.sc", "build.mill", "build.mill.scala")
+    val buildFile = rootBuildFileNames.map(testRepoRoot / _)
+      .find(os.exists)
+      .getOrElse(
+        sys.error(
+          s"No build file named ${rootBuildFileNames.mkString("/")} found in $testRepoRoot"
+        )
+      )
+    for (line <- os.read.lines(buildFile)) {
       val (newState, restOpt) = line match {
         case s"/** Usage" => ("example", None)
         case s"/** See Also: $path */" =>
diff --git a/runner/src/mill/runner/FileImportGraph.scala b/runner/src/mill/runner/FileImportGraph.scala
index fae30b35174..b589c2bfabf 100644
--- a/runner/src/mill/runner/FileImportGraph.scala
+++ b/runner/src/mill/runner/FileImportGraph.scala
@@ -66,7 +66,7 @@ object FileImportGraph {
             val expectedImportSegments = expectedImportSegments0.map(backtickWrap).mkString(".")
             if (
               // Legacy `.sc` files have their package build be optional
-              (s.last.endsWith(".mill") || s.last.endsWith(".mill.sc")) &&
+              (s.last.endsWith(".mill") || s.last.endsWith(".mill.scala")) &&
               expectedImportSegments != importSegments &&
               // Root build.mill file has its `package build` be optional
               !(importSegments == "" && rootBuildFileNames.contains(s.last))
@@ -136,7 +136,7 @@ object FileImportGraph {
           case ImportTree(Seq(("$file", end0), rest @ _*), mapping, start, end) =>
             // Only recursively explore imports from legacy `.sc` files, as new `.mill` files
             // do file discovery via scanning folders containing `package.mill` files
-            if (s.last.endsWith(".sc") && !s.last.endsWith(".mill.sc")) {
+            if (s.last.endsWith(".sc") && !s.last.endsWith(".mill.scala")) {
               val nextPaths = mapping.map { case (lhs, rhs) =>
                 nextPathFor(s, rest.map(_._1) :+ lhs)
               }
@@ -161,10 +161,18 @@ object FileImportGraph {
     }
 
     val rootBuildFiles = rootBuildFileNames
-      .find(rootBuildFileName => os.exists(projectRoot / rootBuildFileName))
-
-    val useDummy = rootBuildFiles.isEmpty
-    val foundRootBuildFileName: String = rootBuildFiles.getOrElse(rootBuildFileNames.head)
+      .filter(rootBuildFileName => os.exists(projectRoot / rootBuildFileName))
+
+    val (useDummy, foundRootBuildFileName) = rootBuildFiles.toSeq match {
+      case Nil => (true, rootBuildFileNames.head)
+      case Seq(single) => (false, single)
+      case multiple =>
+        System.err.println(
+          "Multiple root build files found: " + multiple.mkString(",") +
+            ", picking " + multiple.head
+        )
+        (false, multiple.head)
+    }
 
     val buildFileExtension =
       buildFileExtensions.find(ex => foundRootBuildFileName.endsWith(s".$ex")).get