I admit straight away this is not my idea entirely but I modified it a little to be a sureproof method for anyone wanting this functionality. No Jailbreak required.
1. Get [app]FileAid[/app] (free) on your iPhone and learn how to use it
2. Get Diskaid and run it on your PC/Mac
3. Copy any file to your iPhone using Diskaid, it will create a Start Here folder and this folder will also be visible and available in FileAid. Notice that both DiskAid and FileAid will create their own "Start Here" folder. The one we're interested in is DiskAid's however, as it is for some reason created under iPhone's Camera Roll
4. Now here comes the trick: Any file with the extension .jpg copied to this folder, either via FileAid or DiskAid, will be available for import as soon as you plug your iPhone into any PC:
What this means, you can rename ANY file's extension to .jpg and using this method copy it on your iphone's "usb storage".
5. You may uninstall Diskaid at this point as we won't need it anymore and it's shareware anyway
However, the installation executable I personally renamed to diskaid_3_1.jpg and keep it on my iPhone USB for those cases when you need to copy anything from the target machine to your iPhone.
Also, you may now delete the FileAid's Start Here folder on your iPhone, and in FileAid's settings turn on "Use DiskAid folder for FTP"
6. ???
7. PROFIT
Downsides are two:
1) Copying large files to the phone over WiFi (that is if you decided not to purchase DiskAid which makes USB transfer possible) takes a long time
2) You have to rename the file extension (and possibly also filename if needed) back to what they were originally once the camera import wizard finishes. To make things easier, you can rename as follows: file001.exe --> file001.exe.jpg and vice versa after import is done.
1. Get [app]FileAid[/app] (free) on your iPhone and learn how to use it
2. Get Diskaid and run it on your PC/Mac
3. Copy any file to your iPhone using Diskaid, it will create a Start Here folder and this folder will also be visible and available in FileAid. Notice that both DiskAid and FileAid will create their own "Start Here" folder. The one we're interested in is DiskAid's however, as it is for some reason created under iPhone's Camera Roll
4. Now here comes the trick: Any file with the extension .jpg copied to this folder, either via FileAid or DiskAid, will be available for import as soon as you plug your iPhone into any PC:

What this means, you can rename ANY file's extension to .jpg and using this method copy it on your iphone's "usb storage".
5. You may uninstall Diskaid at this point as we won't need it anymore and it's shareware anyway
Also, you may now delete the FileAid's Start Here folder on your iPhone, and in FileAid's settings turn on "Use DiskAid folder for FTP"
6. ???
7. PROFIT
Downsides are two:
1) Copying large files to the phone over WiFi (that is if you decided not to purchase DiskAid which makes USB transfer possible) takes a long time
2) You have to rename the file extension (and possibly also filename if needed) back to what they were originally once the camera import wizard finishes. To make things easier, you can rename as follows: file001.exe --> file001.exe.jpg and vice versa after import is done.