diff --git a/snowpack/src/build/file-urls.ts b/snowpack/src/build/file-urls.ts
index 827d88c939..eb699d91e5 100644
--- a/snowpack/src/build/file-urls.ts
+++ b/snowpack/src/build/file-urls.ts
@@ -2,27 +2,6 @@ import path from 'path';
 import {MountEntry, SnowpackConfig} from '../types/snowpack';
 import {getExt, replaceExt} from '../util';
 
-export const defaultFileExtensionMapping = {
-  '.mjs': '.js',
-  '.jsx': '.js',
-  '.ts': '.js',
-  '.tsx': '.js',
-  '.vue': '.js',
-  '.svelte': '.js',
-  '.mdx': '.js',
-  '.svx': '.js',
-  '.elm': '.js',
-  '.yaml': '.json',
-  '.toml': '.json',
-  '.php': '.html',
-  '.md': '.html',
-  '.ejs': '.html',
-  '.njk': '.html',
-  '.scss': '.css',
-  '.sass': '.css',
-  '.less': '.css',
-};
-
 /**
  * Map a file path to the hosted URL for a given "mount" entry.
  */
@@ -44,7 +23,7 @@ export function getUrlForFileMount({
     baseExt,
     mountEntry.static
       ? baseExt
-      : config._extensionMap[baseExt] || defaultFileExtensionMapping[baseExt] || baseExt,
+      : config._extensionMap[baseExt] || baseExt,
   );
 }
 
diff --git a/snowpack/src/build/import-resolver.ts b/snowpack/src/build/import-resolver.ts
index 867952da0b..5a4031eb22 100644
--- a/snowpack/src/build/import-resolver.ts
+++ b/snowpack/src/build/import-resolver.ts
@@ -3,7 +3,7 @@ import path from 'path';
 import url from 'url';
 import {ImportMap, SnowpackConfig} from '../types/snowpack';
 import {findMatchingAliasEntry, getExt, replaceExt} from '../util';
-import {defaultFileExtensionMapping, getUrlForFile} from './file-urls';
+import {getUrlForFile} from './file-urls';
 
 const cwd = process.cwd();
 
@@ -32,7 +32,7 @@ function resolveSourceSpecifier(spec: string, stats: fs.Stats | false, config: S
   }
   // Transform the file extension (from input to output)
   const {baseExt} = getExt(spec);
-  const extToReplace = config._extensionMap[baseExt] || defaultFileExtensionMapping[baseExt];
+  const extToReplace = config._extensionMap[baseExt];
   if (extToReplace) {
     spec = replaceExt(spec, baseExt, extToReplace);
   }
diff --git a/snowpack/src/config.ts b/snowpack/src/config.ts
index dbf452c395..e5397eb010 100644
--- a/snowpack/src/config.ts
+++ b/snowpack/src/config.ts
@@ -9,7 +9,6 @@ import {validate, ValidatorResult} from 'jsonschema';
 import os from 'os';
 import path from 'path';
 import yargs from 'yargs-parser';
-import {defaultFileExtensionMapping} from './build/file-urls';
 import {logger} from './logger';
 import {esbuildPlugin} from './plugins/plugin-esbuild';
 import {
@@ -267,9 +266,7 @@ function parseScript(script: string): {scriptType: string; input: string[]; outp
   } else if (cleanInput[0] === '.vue') {
     cleanOutput = ['.js', '.css'];
   } else if (cleanInput.length > 0) {
-    cleanOutput = Array.from(
-      new Set(cleanInput.map((ext) => defaultFileExtensionMapping[ext] || ext)),
-    );
+    cleanOutput = [...cleanInput];
   }
 
   return {