Various issues exist for me here:
1- If you have a group of extensions in one row (example *.jpeg; *.bmp) and you decide you disable that row, if you click at the beginning of the row, it will only disable the *.jpeg by switching to -.jpeg. The *.bmp won't be affected. Also if *.jpeg exists more than once in that group (happened to me by the way), the second *.jpeg won't be affected. It obviously makes sense to group extensions so that you don't duplicate the same behavior for individual extensions (for example, I open *.jpeg; *.bmp using a specific image viewer)
2- Since it is possible to add the extension more than once (either in a group or on it's own), it can be tedious to sort out which extension was matched first from a large group of extensions (see my list above). I think MVV's suggestion is good where you type out the extension and select the desired group or individual extension on it's own. There is also a suggestion I make at the end. Please see below.
3- The thought of using individual extensions per row can be handy as well but only if there is a "duplicate" button to duplicate an existing extension. For example, now I would like to add *.ico. Currently the way I do this is editing WINCMD.INI file (it's tedious and error prone especially when having to increment all these "Filter#_XXXXXX") . It would be handy to have a duplicate button pressed say on *.jpeg so that I don't have to type out the same thing for the newly added *.ico. I find this handy so that I can get around issue #1 by disabling individual extensions since it's now possible to disable them if they're not the first entry in a group. You will probably say "Just create a new entry called "*.ico" and move it to the top. Not a clean approach.
4- Sometimes I would like to merge some behaviour between extensions. For example, I would like certain extensions to have an encryption capability or "send to dropbox". Currently I have to edit WINCMD.INI and edit the necessary extensions.
** just a thought. Can the list be a group of extensions (like shown below). For example, an "Images" group having a list of graphic extensions that can be easily controlled by selecting the group itself. Something like this:
If you click on "Images", it will affect all contained extensions. For example clicking on Images will toggle it to -Images, which will disable all contained extensions (*.jpeg, *.bmp; *.ico), etc. If I decide to disable only *.ico, I can only click on that without affecting the other graphic extensions in that group. if I decide to handle *.ico separately, I can drag it out of "Images" on to its own row. Also, If I would like to copy some actions that exist only in *.doc for example to other extensions, I can right click on *.doc, display current actions for *.doc, select a particular action or group of actions, click copy. Then I can go to individual extensions and click paste.