Ok, a quick answer then :)
Here is an example from an upload we made at Wikimedia France in september. Metadata matching is done against the last column of the CSV file, which contains the exact name of the picture file. It could be any information available in the EXIF as well.
In ComeOn!, in the "open pictures" dialog, first pick the CSV file. The combo box will update with the column titles from the file (there is a bug here that won't refresh the combo when you select a file again without closing the dialog). Fill in the right field the expression that selects the field from the picture metadata to match. It may be any EXIF field (as show in the left pane of the main window when displaying a picture) or a special value from a reserved keyword (I need to document these), as "
file.name".
The "regular expression" and "substitution" fields may be used to transform the picture metadata before matching it against the file. The default is safe as it keeps the information as is, but you could use it to extract a part of the "fichier" column, add a prefix and a suffix and so on.
In the file description templates, metadata from the file may be inserted with this stance: "$external." appended with the CSV column title. After loading the pictures, if the matching was succesful, the "content" tab should show a proper page.
BR,
Édouard