As the title says, it would be useful to allow both borrowed and allocated types for a file mask.

Without looking at the code, my assumption is this should be fairly easy to accomplish just by replacing the field mask: &'static str with mask: Cow<str> and internally lower it to an &str. Library users would have to make a breaking change to add an into() call to their static masks

Example usecase

For lutgen, it would be nice to keep a static slice of file extensions directly, used for both creating a custom footer doc and the file mask. ATM, I have to define the mask itself (ie, *.png|*.jpg) and then for the doc remove *. and split the string along | chars to be able to use doc.literal() for each item. With this change, I can move my constant to just a slice, and construct the mask at runtime