Nemo Preview PNG loading Bug

nemo-preview: PNG files on CIFS/SMB shares remain stuck on "Loading..."

Distribution

Linux Mint 22.3 Zena, Cinnamon

Package versions

nemo:
  Installed: 6.6.3+zena

nemo-preview:
  Installed: 6.6.0+zena

cjs:
  Installed: 115.1+zena

Bug description

Nemo Preview fails to display PNG files located on a network share.

When a PNG file on a CIFS-mounted NAS share is selected in Nemo and the Spacebar is pressed, the Nemo Preview window opens but remains indefinitely on:

Loading...

JPEG files in the same directory on the same network share preview correctly.

The same PNG file works correctly when copied to a local filesystem such as /tmp and previewed from Nemo using the Spacebar.

The issue can also be reproduced when accessing the NAS through smb:// in Nemo instead of the kernel CIFS mount.

Network mount

The share is mounted using CIFS:

//ds725_nas.local/home on /mnt/nas-home type cifs

Relevant mount options include:

rw,relatime,vers=3.0,cache=strict,
uid=1000,forceuid,gid=1000,forcegid,
soft,nounix,serverino,mapposix,reparse=nfs,
nativesocket,rsize=4194304,wsize=4194304,
actimeo=1,_netdev,x-systemd.automount

Steps to reproduce

1. Mount an SMB/CIFS share.
2. Open the share in Nemo.
3. Select a PNG file.
4. Press Spacebar.
5. Nemo Preview opens but remains on "Loading...".
6. Select a JPEG file in the same directory.
7. Press Spacebar.
8. The JPEG preview works correctly.
9. Copy the PNG file to /tmp.
10. Open /tmp in Nemo and press Spacebar on the PNG.
11. The PNG preview now works correctly.

The same problem also occurs when accessing the share directly through:

smb://server/share

Expected behavior

PNG files on SMB/CIFS shares should preview in the same way as PNG files stored locally and JPEG files stored on the same network share.

Actual behavior

PNG files on the network share remain indefinitely on "Loading...".

Relevant journal output

When attempting to preview the PNG from Nemo:

GFileInfo created without standard::content-type

file ../../../gio/gfileinfo.c: line 1821
(g_file_info_get_content_type): should not be reached

JS ERROR: Error calling prepare() on viewer:
Error: Argument type may not be null

getObject@/usr/share/nemo-preview/js/ui/mimeHandler.js:74:25
_createRenderer/<@/usr/share/nemo-preview/js/ui/mainWindow.js:332:71
run@/usr/share/nemo-preview/js/ui/main.js:57:24
@../src/main.c:2:6

The relevant code in mainWindow.js is:

this._pendingRenderer =
    this._mimeHandler.getObject(this._fileInfo.get_content_type());

and mimeHandler.js subsequently calls:

Gio.content_type_is_a(mime, key)

where mime appears to be null.

MIME type tests

Outside nemo-preview, GIO correctly identifies the network PNG:

$ gio info -a standard::content-type "CCC.png"

attributes:
  standard::content-type: image/png

xdg-mime also identifies the same PNG correctly:

$ xdg-mime query filetype /tmp/CCC.png
image/png

A JPEG on the same CIFS share is also correctly identified:

$ gio info -a standard::content-type "foto 1.jpg"

attributes:
  standard::content-type: image/jpeg

PNG decoding test

GdkPixbuf can load the PNG without problems:

from gi.repository import GdkPixbuf
p = GdkPixbuf.Pixbuf.new_from_file("/tmp/CCC.png")
print(p.get_width(), p.get_height())

Output:

4032 3024

The PNG is also correctly identified by file:

PNG image data, 4032 x 3024, 8-bit/color RGBA, non-interlaced

Additional reproduction test

A JPEG that previews correctly was converted locally to a new PNG using GdkPixbuf.

The resulting file:

PNG image data, 2448 x 3264, 8-bit/color RGB, non-interlaced

behaves as follows:

• /tmp/test.png → Spacebar preview works
• same test.png copied to the CIFS share → preview remains on "Loading..."

This suggests the problem is not caused by the contents or metadata of a particular PNG.

Troubleshooting already performed

• Reinstalled nemo-preview.
• Logged out/in.
• Tested multiple PNG files.
• Tested newly generated PNG.
• Tested local filesystem versus CIFS.
• Tested kernel CIFS mount.
• Tested direct smb:// access through Nemo.
• Verified GdkPixbuf PNG decoding.
• Verified GIO MIME detection.
• Verified xdg-mime detection.
• Verified JPEG preview works on the same network share.

The problem appears to occur specifically when nemo-preview asynchronously retrieves the file information for a PNG located on an SMB/CIFS resource. GFileInfo.get_content_type() unexpectedly returns null even though GIO correctly reports image/png when queried separately.

Start with nemo-preview/js/ui/mainWindow.js and nemo-preview/js/ui/mimeHandler.js, focusing on the asynchronous file-information path and the null value passed to Gio.content_type_is_a(). Reproduce the issue with a PNG on CIFS/SMB and compare it with a local or JPEG preview. Done means PNG previews no longer remain on "Loading..." and the null content-type error is resolved.