fix(xo-server/rest-api): fix broken VDI export

Introduced by 7b256ab

Not completely fixed by 8eae8e7

See zammad#27167

CHANGELOG.unreleased.md

@@ -21,6 +21,7 @@
 
 - [Hub/Recipes/Kubernetes] Properly sort versions in selector
 - [Host/Network] Fix `an error has occurred` briefly displaying in 'Mode' column of the host's Network tab (PR [#7954](https://github.com/vatesfr/xen-orchestra/pull/7954))
+- [REST API] Fix VDI export broken in XO 5.96.0 and not completely fixed in XO 5.98.0
 
 ### Packages to release
 
@@ -41,6 +42,7 @@
 - @xen-orchestra/lite minor
 - @xen-orchestra/web minor
 - @xen-orchestra/web-core minor
+- xo-server patch
 - xo-web minor
 
 <!--packages-end-->

packages/xo-server/src/xo-mixins/rest-api.mjs

@@ -955,7 +955,14 @@ export default class RestApi {
         const nbdConcurrency = req.query.nbdConcurrency && parseInt(req.query.nbdConcurrency)
         const stream = await req.xapiObject.$exportContent({ format: req.params.format, preferNbd, nbdConcurrency })
 
-        res.writeHead(200, 'OK', { 'content-disposition': 'attachment', 'content-length': stream.length })
+        const headers = { 'content-disposition': 'attachment' }
+
+        const { length } = stream
+        if (length !== undefined) {
+          headers['content-length'] = length
+        }
+
+        res.writeHead(200, 'OK', headers)
         await pipeline(stream, res)
       })
     )