Please help, I lost an important file

Hello, I have lost an important movie file (.mp4) and I don't know what I did, but I think it has something to do with Aliases.

A few months ago, to clear out space on my MBA, I copied this file (2.4GB) to my Seagate External hard drive. I did not know whether it was copied as an Alias back then. I then permanently deleted the original file on my MBA. (I use a Finder-like program called MuCommander)

Today, I wanted to copy the movie file from my Seagate hard drive to one of my SD cards. (using MuCommander) I found that the file would not get copied, and that's when I realised there was a tiny arrow on the icon. I now know it is an Alias, but I did not know about it at the time. Subsequently, I tried double clicking the file and found that the movie could still be played, I am certain about that. I then tried to rename the extension of the file from .mp4 to something else, thinking that maybe that way I could copy the file to my SD card. Unfortunately I accidentally deleted the three letters "mp4" and pressed enter and the alias file vanished. (to be more clear, let's say the file was abcd.mp4, I accidentally renamed it to abcd. and pressed enter by mistake)

After this I started searching for answers online and that's when I found out what alias files are. But, now I've accidentally caused the alias file to vanish and I cannot find the original.

Since I could play the movie earlier on, I believe that the original still exists somewhere, but, I cannot find it. As I am more familiar with windows, I have tried inserting my hard drive to a WinXP machine and did a search, but I cannot find the original.

Any help would be greatly appreciated.

Thank you!