diff --git a/index.html b/index.html
index f4431f8..ddbf8c1 100644
--- a/index.html
+++ b/index.html
@@ -73,12 +73,13 @@
             <p>
               You can request your Mastodon archive by logging into your account
               on the web, then visiting "Preferences > Import and export >
-              Request your archive".
-            </p>
-            <p>
-              <strong>Start by opening your archive file with MARL.</strong>
+              Request your archive".<br />
+              <strong>Please note:</strong> only ZIP files are supported (not
+              GZ).
             </p>
             <p>
+              <strong>Start by opening your archive file with MARL.</strong
+              ><br />
               You can drag and drop it anywhere on this page, or
               <label for="file-loader" tabindex="0"
                 >click here to select it</label
@@ -89,7 +90,7 @@
               id="file-loader"
               class="file-loader"
               type="file"
-              accept=".zip, .gz"
+              accept=".zip"
               x-show="!$store.files.loading"
               @change="unZip(Object.values($event.target.files))"
             />
diff --git a/readme.md b/readme.md
index 5d4e563..fd57b64 100644
--- a/readme.md
+++ b/readme.md
@@ -12,6 +12,8 @@ __Access MARL online here:__ https://s427.github.io/MARL/
 
 Alternatively, you can download this project from its [project page](https://github.com/s427/MARL), unpack it on your device, and simply launch (double click) `index.html`.
 
+⚠️ __Only ZIP files are supported__ —not GZ files. This is a limitation of the JSZip library which is used by MARL to unpack the archive file. Older versions of Mastodon (prior to version 4.2.0) used to export the archive in TAR.GZ. If this is your case, you will have to first convert your archive to a ZIP file.
+
 ## Screenshot
 
 <img src="screenshot.png" alt="app screenshot">