I recently fresh installed Linux Mint 12 (Lisa), which is based on Ubuntu 11.10 (Oneiric). I almost immediately bumped into a problem with gdebi, the graphical package installer. The message I received was:

"Could not open XXXX.deb.
The package might be corrupted or you are not allowed to open the file.
Check the permissions of the file."

After a lot of searching, I found this solution in linuxmint forums. Works like a charm:

[code]sudo apt-get install xz-lzma[/code]

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