Added support for PNG images (both for downloading/saving and loading).
Switched back to using QFile instead of QImage for saving as QImage's save function adds overhead to the file size, leaving me unsure whether the original image is affected.
Refactored the magic bytes into two QLists: one containing a list of QStrings representing the supported extensions, the other containing a list of QByteArrays representing the magic bytes in hex.
Refactored the image loading loop to be two nested for loops: the outer loop iterating through the QList of paths to folders in which to search for images and the inner loop iterating through the QList of supported extensions.
Removed the QLists for determining image format. Instead, using QImageReader, both when downloading/saving and when loading, to determine the correct format (Cockatrice now supports all QImageReader-supported formats).
Image loading still uses one for loop to iterate through the QList of paths to folders in which to search for images.
Support loading of files with wrong extension

Like M15’s .png that cockatrice wrongly saved as .jpg
Additionally, move the QImageReader declaration outside of the inner
loop
Merge branch 'ctrlaltca-pr256' into handle-PNGs-2
Merge pull request #256 from pliu037/handle-PNGs-2

Handle PNGs (and other image formats)