diff --git a/web/src/App.svelte b/web/src/App.svelte
index 62a02b8..9229ebc 100644
--- a/web/src/App.svelte
+++ b/web/src/App.svelte
@@ -4,6 +4,7 @@
   import { parseRoute, buildHash, currentHash } from './lib/route.js'
   import MarkdownEditor from './lib/MarkdownEditor.svelte'
   import ExcelEditor from './lib/ExcelEditor.svelte'
+  import { isSpreadsheetName } from './lib/spreadsheet.js'
 
   let user = $state(null)
   let loading = $state(true)
@@ -398,10 +399,6 @@
     return `${(n / (1024 * 1024)).toFixed(1)} MB`
   }
 
-  function isExcelName(name) {
-    return /\.xlsx$/i.test(name || '')
-  }
-
   async function onAttachFile(e) {
     const file = e.target?.files?.[0]
     e.target.value = ''
@@ -963,7 +960,7 @@
                     <span class="muted">{formatBytes(a.sizeBytes)}</span>
                   </div>
                   <div class="row" style="width: auto; justify-content: flex-end;">
-                    {#if isExcelName(a.filename)}
+                    {#if isSpreadsheetName(a.filename)}
                       <button class="primary" onclick={() => (excelAtt = a)}>Edit</button>
                     {/if}
                     <a
diff --git a/web/src/lib/ExcelEditor.svelte b/web/src/lib/ExcelEditor.svelte
index 2e5976c..fa20155 100644
--- a/web/src/lib/ExcelEditor.svelte
+++ b/web/src/lib/ExcelEditor.svelte
@@ -1,5 +1,6 @@
 <script>
   import { api } from './api.js'
+  import { spreadsheetWriteOptions } from './spreadsheet.js'
 
   let {
     projectId,
@@ -87,11 +88,9 @@
     try {
       await ensureXlsx()
       workbook.Sheets[activeSheet] = gridToSheet(grid)
-      const out = XLSX.write(workbook, { bookType: 'xlsx', type: 'array' })
-      await api.putAttachment(projectId, ticketId, attachment.id, out, {
-        contentType:
-          'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
-      })
+      const { bookType, contentType } = spreadsheetWriteOptions(attachment.filename)
+      const out = XLSX.write(workbook, { bookType, type: 'array' })
+      await api.putAttachment(projectId, ticketId, attachment.id, out, { contentType })
       onsaved()
       msgOk()
     } catch (e) {
diff --git a/web/src/lib/spreadsheet.js b/web/src/lib/spreadsheet.js
new file mode 100644
index 0000000..1cd15ad
--- /dev/null
+++ b/web/src/lib/spreadsheet.js
@@ -0,0 +1,45 @@
+/** Extensions SheetJS can read/write for simple cell editing. */
+const FORMATS = {
+  xlsx: {
+    bookType: 'xlsx',
+    contentType: 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
+  },
+  xlsm: {
+    bookType: 'xlsm',
+    contentType: 'application/vnd.ms-excel.sheet.macroEnabled.12',
+  },
+  xlsb: {
+    bookType: 'xlsb',
+    contentType: 'application/vnd.ms-excel.sheet.binary.macroEnabled.12',
+  },
+  xls: {
+    bookType: 'xls',
+    contentType: 'application/vnd.ms-excel',
+  },
+  ods: {
+    bookType: 'ods',
+    contentType: 'application/vnd.oasis.opendocument.spreadsheet',
+  },
+  fods: {
+    bookType: 'fods',
+    contentType: 'application/vnd.oasis.opendocument.spreadsheet',
+  },
+  csv: {
+    bookType: 'csv',
+    contentType: 'text/csv',
+  },
+}
+
+const EXT_RE = /\.([a-z0-9]+)$/i
+
+export function isSpreadsheetName(name) {
+  const m = String(name || '').match(EXT_RE)
+  if (!m) return false
+  return Object.prototype.hasOwnProperty.call(FORMATS, m[1].toLowerCase())
+}
+
+export function spreadsheetWriteOptions(filename) {
+  const m = String(filename || '').match(EXT_RE)
+  const ext = m ? m[1].toLowerCase() : 'xlsx'
+  return FORMATS[ext] || FORMATS.xlsx
+}