A simple way to find assets by file extension is to perform a keyword search by typing in the wildcard character followed by a dot and the extension you want into the search field:
Another option is to configure a dropdown field (you need to be an admin user to do this):
Go to Admin-> Fields and click "Add new"
Enter the title of the new field, e.g. "File Extension"
Enter the values you want, for example "JPG", "PNG", "EPS", etc
Click "Populate with data embedded in the file", enter "FileTypeExtension" and hit return to add this as a mapping.
Go to Admin->Filters and add the new field to the filter so users can filter by this.
Now, when a new file is uploaded, the value of this field will be set automatically depending on the extension of the file.
Some limitations of this approach:
You can't map multiple file extensions to one single value in the dropdown, so (for example) JPEG and JPG would need to be separate entries (if you have any files with .jpeg extensions).
The values have to exist already in the dropdown - they won't be added automatically as files are uploaded. So you need to try to think of all your extensions and add them when you create the field.
It's best to create this new field before you upload or import your files into Dash as the values will not be set automatically for assets that are already in Dash, only for uploads from that point on. You get round this you could do bulk updates (using the keyword search above) to find all the assets for a particular file extension and then manually set the dropdown value.