Why This Happens
Sometimes a downloaded installation file (a disk image such as .img or .iso) won't mount, and you'll see an error saying the file can't be mounted. This usually means the download was interrupted, partially blocked by the system, or the file's metadata got corrupted during the download or transfer, even though the file itself is otherwise intact.
Step 1: Try the Standard Fix First
Before anything else, try these standard steps:
Delete the current file from your Downloads folder.
Restart your computer completely (not just the virtual machine, if you're running Windows through Parallels or similar software).
Download the file again, but don't open it directly from the browser.
Open your Downloads folder, right-click the file, and choose Open with > File Explorer (this mounts it as a virtual DVD drive). Inside, double-click Setup to start installing.
Step 2: Compress the File Into a ZIP
If the file still won't mount after trying the steps above, compressing it into a ZIP archive and opening it from there resolves the issue in most cases:
Right-click the downloaded installation file and choose Compress (or Send to > Compressed (zipped) folder) to create a ZIP file.
Open the new ZIP file.
Run the installer from inside the ZIP, or extract it first and run it from the extracted folder.
Re-compressing the file this way effectively rebuilds it, which clears whatever was blocking it from mounting normally.