Show and enforce max upload size in the web UI with clear error messages

index.html.tmpl

@@ -93,6 +93,7 @@
         <h2>Upload with curl</h2>
         <code>curl --upload-file myfile.txt {{ .Proto }}://{{ .Host }}/myfile.txt</code>
         <p style="margin-top: 10px;">The server will respond with a download URL.</p>
+        <p style="margin-top: 10px;">Maximum upload size: <strong id="maxUploadSizeLabel" data-max-bytes="{{ .MaxUploadSize }}">{{ .MaxUploadSize }} bytes</strong></p>
     </div>
 
     <div class="section">
@@ -105,6 +106,7 @@
         <h2>Web Upload</h2>
         <div class="upload-area" id="uploadArea">
             <p>📁 Click to select files or drag and drop here</p>
+            <p style="margin-top: 10px; font-size: 0.85em; color: #666;">Maximum file size: <span id="maxUploadSizeText"></span></p>
             <input type="file" id="fileInput" multiple>
             <button onclick="document.getElementById('fileInput').click()">Select Files</button>
         </div>
@@ -120,6 +122,22 @@
         const progressContainer = document.getElementById('progressContainer');
         const progressBar = document.getElementById('progressBar');
         const results = document.getElementById('results');
+        const maxUploadSize = parseInt(document.getElementById('maxUploadSizeLabel').dataset.maxBytes, 10) || 0;
+
+        function formatBytes(bytes) {
+            if (!bytes) return 'unknown';
+            const units = ['B', 'KB', 'MB', 'GB', 'TB'];
+            let value = bytes;
+            let unitIndex = 0;
+            while (value >= 1024 && unitIndex < units.length - 1) {
+                value /= 1024;
+                unitIndex++;
+            }
+            return value.toFixed(unitIndex === 0 ? 0 : 1) + ' ' + units[unitIndex];
+        }
+
+        document.getElementById('maxUploadSizeLabel').textContent = formatBytes(maxUploadSize);
+        document.getElementById('maxUploadSizeText').textContent = formatBytes(maxUploadSize);
 
         uploadArea.addEventListener('click', () => fileInput.click());
 
@@ -146,6 +164,27 @@
         function uploadFiles(files) {
             if (files.length === 0) return;
 
+            const oversizedFiles = [];
+            let totalSize = 0;
+            for (let file of files) {
+                totalSize += file.size;
+                if (maxUploadSize && file.size > maxUploadSize) {
+                    oversizedFiles.push(file.name);
+                }
+            }
+
+            if (oversizedFiles.length > 0) {
+                results.innerHTML = '<div class="result-item">Upload failed: the following file(s) exceed the maximum size of ' +
+                    formatBytes(maxUploadSize) + ': ' + oversizedFiles.join(', ') + '</div>';
+                return;
+            }
+
+            if (maxUploadSize && totalSize > maxUploadSize) {
+                results.innerHTML = '<div class="result-item">Upload failed: total upload size (' + formatBytes(totalSize) +
+                    ') exceeds the maximum of ' + formatBytes(maxUploadSize) + '</div>';
+                return;
+            }
+
             const formData = new FormData();
             for (let file of files) {
                 formData.append('files', file);

main.go

@@ -430,7 +430,11 @@ func logRequest(r *http.Request, status int, duration time.Duration) {
 func serveFrontpage(w http.ResponseWriter, r *http.Request) {
 	proto := getProtocol(r)
 	host := getHost(r)
-	pageData := map[string]string{"Proto": proto, "Host": host}
+	pageData := map[string]any{
+		"Proto":         proto,
+		"Host":          host,
+		"MaxUploadSize": maxUploadSize,
+	}
 
 	w.Header().Set("Content-Type", "text/html; charset=utf-8")
 	indexHTML.Execute(w, pageData)