Steps to reproduce:

1. Open "Repository" menu
2. Click "Git LFS" sub-menu
3. Click "Track / Untrack files" item
4. Click the "+" button to add one entry to the list
   (in my case the list starts empty, but I don't think it makes any difference)
5. A call-out box appears where the user is asked to either type the extension of the files to be tracked with Git LFS, or to choose it from a drop-down menu.
6. DON'T open the drop-down menu, only type the extension (for example "lib", or "*.lib")
7. Click the OK button

Expected result:

The new extension should be added to the list

Actual result:

A dialog window appears complaining that "You must type or select an extension to add.", even if the extension was actually typed.

Additional notes:

While writing this report I noticed two workarounds:

1. At step #7, if instead of clicking the "OK" button I press the return key on the keyboard, then the extension is correctly accepted and added to the list as expected.
2. At step #6, if I open and then close the drop down menu before typing the extension, even without selecting anything from the list, then pressing the "OK" button correctly adds the typed extension to the list, as expected