Correctly detect if image format is supported by QImageWriter

The code queries potential image formats by stripping a mimetype of its
'image/' prefix and making the rest of the mimetype capitalised, such as
'image/png' -> 'PNG'. The problem is that this is then searched for in
QImageWriter::supportedImageFormats() by simple equality. The method
returns a list of lowercase byte arrays, not uppercase. As the codepath
can never match due to checking for an uppercase word in an array of
lowercase words, this means that images are effectively always sent as
BMP format, even if they should be sent in other formats, such as PNG
or JPEG.

A simple inspection with GDB (or a qDebug) reveals this:

```
(gdb) p QImageWriter::supportedImageFormats()
$31 = {"bmp" = {...}, "bw" = {...}, "cur" = {...}, "eps" = {...},
  "epsf" = {...}, "epsi" = {...}, "icns" = {...},
  "ico" = {...}, "jp2" = {...}, "jpeg" = {...}, "jpg" = {...},
  "pbm" = {...}, "pcx" = {...}, "pgm" = {...},
  "pic" = {...}, "png" = {...}, "ppm" = {...},
  "rgb" = {...}, "rgba" = {...}, "sgi" = {...},
  "tga" = {...}, "tif" = {...}, "tiff" = {...},
  "wbmp" = {...}, "webp" = {...}, "xbm" = {...}, "xpm" = {...}}
```

```
(gdb) p QImageWriter::supportedImageFormats().contains("PNG")
$32 = false
```

```
(gdb) p QImageWriter::supportedImageFormats().contains("png")
$33 = true
```

The fix for this is simple: lowercase the remainder of the mimetype,
instead of uppercasing it, and we can start hitting the codepath that's
supposed to write non-BMP formats.

Change-Id: Id3e9b730b7edcabcb2f1b04d8ef0a4c1fb9c9159
Reviewed-by: David Edmundson <davidedmundson@kde.org>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/plugins/platforms/wayland/shared/qwaylandmimehelper.cpp

@@ -60,7 +60,7 @@ QByteArray QWaylandMimeHelper::getByteArray(QMimeData *mimeData, const QString &
             buf.open(QIODevice::ReadWrite);
             QByteArray fmt = "BMP";
             if (mimeType.startsWith(QLatin1String("image/"))) {
-                QByteArray imgFmt = mimeType.mid(6).toUpper().toLatin1();
+                QByteArray imgFmt = mimeType.mid(6).toLower().toLatin1();
                 if (QImageWriter::supportedImageFormats().contains(imgFmt))
                     fmt = imgFmt;
             }