It would be great to allow unrestricted file extensions.
I want to import into Hydrus many 3D models (.stl; .step; .prt; .x_t; .obj; et cetera) and some others like .txt and .html.
Viewers for non supported files are not necessary if a custom screenshot (provided by the user) of the file content is showed in Hydrus.
Then the workflow would be to import, to tag, to add a screenshot, to search, and then to execute with an external program.