Canon EOS-1D C Manual (en)

Changing Settings
Configured in [FTP server]  [Directory structure].
Selecting [Camera] automatically creates a folder structure matching 
that of the camera’s (such as A/DCIM/100EOS1D) in the server’s root 
folder for image storage. If you have created a subfolder in the root 
folder by changing the [Target folder] setting, a folder structure such as 
A/DCIM/100EOS1D is automatically created in that folder for image 
Selecting [Default] will use the root folder for image storage. If you have 
created a subfolder in the root folder by changing the [Target folder
setting, images are saved in that folder.
Configured in [FTP server]  [Overwrite same file].
When the camera is configured to prevent overwriting
If there is already a file of the same name in the target folder on the FTP 
server, a suffix consisting of an underscore and a number is appended 
to the new file’s name. For example: IMG_0003_1.JPG.
When you resend images if initial transfer fails
Even if the camera is configured to overwrite files of the same name, if 
you resend an image file that could not be transferred initially, the 
existing file may not be overwritten in some cases. If this happens, a 
suffix consisting of an underscore, a letter, and a number is appended 
to the new file’s name. For example: IMG_0003_a1.JPG.
Directory Structure of the Target Folder
Overwriting Files of the Same Name