Transparency problem in thumbnails of PNG files

**Describe the bug**
A clear and concise description of what the bug is.
- Which site: both
-
**To Reproduce**
Steps to reproduce the behavior:
1. Open a post with a PNG file with transparency
2. The areas that should be transparent are grey

**Expected behavior**
Transparency handling in the thumbnail should be the same as in full size after you click on the thumbnail.

**Screenshots**
Attached screenshots from Phanpy dev version and the default Mastodon web client.

**Desktop (please complete the following information):**
- OS: Linux
- Browsers: Firefox 111 and Web (Epiphany) 44.

**Smartphone (please complete the following information):**
- Device: Moto G7
- OS: Android
- Browser: Firefox
- Version 111