diff --git a/cli/lsp/completions.rs b/cli/lsp/completions.rs
index 7a93c8baf1..73f772cf22 100644
--- a/cli/lsp/completions.rs
+++ b/cli/lsp/completions.rs
@@ -526,7 +526,7 @@ mod tests {
       documents.open(
         specifier.clone(),
         *version,
-        language_id.clone(),
+        *language_id,
         (*source).into(),
       );
     }
diff --git a/cli/lsp/diagnostics.rs b/cli/lsp/diagnostics.rs
index 58156bad26..2723b44dbd 100644
--- a/cli/lsp/diagnostics.rs
+++ b/cli/lsp/diagnostics.rs
@@ -1010,7 +1010,7 @@ mod tests {
       documents.open(
         specifier.clone(),
         *version,
-        language_id.clone(),
+        *language_id,
         (*source).into(),
       );
     }
diff --git a/cli/lsp/documents.rs b/cli/lsp/documents.rs
index 308e987bdb..edf5497aba 100644
--- a/cli/lsp/documents.rs
+++ b/cli/lsp/documents.rs
@@ -80,7 +80,7 @@ static TSX_HEADERS: Lazy<HashMap<String, String>> = Lazy::new(|| {
     .collect()
 });
 
-#[derive(Debug, Clone, PartialEq, Eq)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum LanguageId {
   JavaScript,
   Jsx,
@@ -93,6 +93,31 @@ pub enum LanguageId {
 }
 
 impl LanguageId {
+  pub fn as_media_type(&self) -> MediaType {
+    match self {
+      LanguageId::JavaScript => MediaType::JavaScript,
+      LanguageId::Jsx => MediaType::Jsx,
+      LanguageId::TypeScript => MediaType::TypeScript,
+      LanguageId::Tsx => MediaType::Tsx,
+      LanguageId::Json => MediaType::Json,
+      LanguageId::JsonC => MediaType::Json,
+      LanguageId::Markdown | LanguageId::Unknown => MediaType::Unknown,
+    }
+  }
+
+  pub fn as_extension(&self) -> Option<&'static str> {
+    match self {
+      LanguageId::JavaScript => Some("js"),
+      LanguageId::Jsx => Some("jsx"),
+      LanguageId::TypeScript => Some("ts"),
+      LanguageId::Tsx => Some("tsx"),
+      LanguageId::Json => Some("json"),
+      LanguageId::JsonC => Some("jsonc"),
+      LanguageId::Markdown => Some("md"),
+      LanguageId::Unknown => None,
+    }
+  }
+
   fn as_headers(&self) -> Option<&HashMap<String, String>> {
     match self {
       Self::JavaScript => Some(&JS_HEADERS),
@@ -394,7 +419,7 @@ impl Document {
     Ok(Document(Arc::new(DocumentInner {
       specifier: self.0.specifier.clone(),
       fs_version: self.0.fs_version.clone(),
-      maybe_language_id: self.0.maybe_language_id.clone(),
+      maybe_language_id: self.0.maybe_language_id,
       dependencies,
       text_info,
       line_index,
@@ -464,10 +489,22 @@ impl Document {
 
   pub fn media_type(&self) -> MediaType {
     if let Some(Ok(module)) = &self.0.maybe_module {
-      module.media_type
-    } else {
-      MediaType::from(&self.0.specifier)
+      return module.media_type;
     }
+    let specifier_media_type = MediaType::from(&self.0.specifier);
+    if specifier_media_type != MediaType::Unknown {
+      return specifier_media_type;
+    }
+
+    self
+      .0
+      .maybe_language_id
+      .map(|id| id.as_media_type())
+      .unwrap_or(MediaType::Unknown)
+  }
+
+  pub fn maybe_language_id(&self) -> Option<LanguageId> {
+    self.0.maybe_language_id
   }
 
   /// Returns the current language server client version if any.
diff --git a/cli/lsp/language_server.rs b/cli/lsp/language_server.rs
index a97b3d1f2b..e23db159a8 100644
--- a/cli/lsp/language_server.rs
+++ b/cli/lsp/language_server.rs
@@ -1212,6 +1212,14 @@ impl Inner {
       }
       Some(Err(err)) => Err(anyhow!("{}", err)),
       None => {
+        // the file path is only used to determine what formatter should
+        // be used to format the file, so give the filepath an extension
+        // that matches what the user selected as the language
+        let file_path = document
+          .maybe_language_id()
+          .and_then(|id| id.as_extension())
+          .map(|ext| file_path.with_extension(ext))
+          .unwrap_or(file_path);
         // it's not a js/ts file, so attempt to format its contents
         format_file(&file_path, &document.content(), &fmt_options)
       }
diff --git a/cli/lsp/tsc.rs b/cli/lsp/tsc.rs
index c6b5315e06..de34c7fa6a 100644
--- a/cli/lsp/tsc.rs
+++ b/cli/lsp/tsc.rs
@@ -3466,7 +3466,7 @@ mod tests {
       documents.open(
         specifier.clone(),
         *version,
-        language_id.clone(),
+        *language_id,
         (*source).into(),
       );
     }
diff --git a/cli/tests/lsp_tests.rs b/cli/tests/lsp_tests.rs
index 066f3efa81..8ed6a67d71 100644
--- a/cli/tests/lsp_tests.rs
+++ b/cli/tests/lsp_tests.rs
@@ -5339,7 +5339,9 @@ mod lsp {
         "textDocument/didOpen",
         json!({
           "textDocument": {
-            "uri": "file:///a/file.json",
+            // Also test out using a non-json file extension here.
+            // What should matter is the language identifier.
+            "uri": "file:///a/file.lock",
             "languageId": "json",
             "version": 1,
             "text": "{\"key\":\"value\"}"
@@ -5353,7 +5355,7 @@ mod lsp {
         "textDocument/formatting",
         json!({
             "textDocument": {
-              "uri": "file:///a/file.json"
+              "uri": "file:///a/file.lock"
             },
             "options": {
               "tabSize": 2,