diff --git a/src/web/OutputWaiter.mjs b/src/web/OutputWaiter.mjs
index d3cbc8c7..8e21c2d3 100755
--- a/src/web/OutputWaiter.mjs
+++ b/src/web/OutputWaiter.mjs
@@ -514,12 +514,9 @@ class OutputWaiter {
             fileName += ".zip";
         }
 
-        let fileExt = window.prompt("Please enter a file extension for the files: ", ".txt");
+        let fileExt = window.prompt("Please enter a file extension for the files, or leave blank to detect automatically.", "");
 
-        if (fileExt === null) {
-            // Use .dat as the default file extension
-            fileExt = ".dat";
-        }
+        if (fileExt === null) fileExt = "";
 
         if (this.zipWorker !== null) {
             this.terminateZipWorker();
@@ -1076,7 +1073,7 @@ class OutputWaiter {
             sliceToEl = document.getElementById("output-file-slice-to"),
             sliceFrom = parseInt(sliceFromEl.value, 10),
             sliceTo = parseInt(sliceToEl.value, 10),
-            str = Utils.arrayBufferToStr(this.getActive(true).slice(sliceFrom, sliceTo));
+            str = Utils.arrayBufferToStr(this.getActive(false).slice(sliceFrom, sliceTo));
 
         outputText.classList.remove("blur");
         showFileOverlay.style.display = "block";
diff --git a/src/web/ZipWorker.mjs b/src/web/ZipWorker.mjs
index 039b1e32..6af603cc 100644
--- a/src/web/ZipWorker.mjs
+++ b/src/web/ZipWorker.mjs
@@ -8,6 +8,7 @@
 
 import zip from "zlibjs/bin/zip.min";
 import Utils from "../core/Utils";
+import {detectFileType} from "../core/lib/FileType";
 
 const Zlib = zip.Zlib;
 
@@ -48,7 +49,7 @@ self.zipFiles = function(outputs, filename, fileExtension) {
 
     for (let i = 0; i < inputNums.length; i++) {
         const iNum = inputNums[i];
-        const name = Utils.strToByteArray(iNum + fileExtension);
+        let ext = fileExtension;
 
         let output;
         if (outputs[iNum].data === null) {
@@ -58,6 +59,18 @@ self.zipFiles = function(outputs, filename, fileExtension) {
         } else {
             output = new Uint8Array(outputs[iNum].data.dish.value);
         }
+
+        if (fileExtension === "") {
+            // Detect automatically
+            const types = detectFileType(output);
+            if (!types.length) {
+                ext = ".dat";
+            } else {
+                ext = `.${types[0].extension.split(",", 1)[0]}`;
+            }
+        }
+        const name = Utils.strToByteArray(iNum + ext);
+
         zip.addFile(output, {filename: name});
     }