Once you have your Siril's format .fit files, you can switch to the sequence tab seen below. Sequences are what Siril uses to represent the manipulated files. Three file types are natively supported: a set of two or more FITS files, a single SER file or, to some extent, a regular film file such as AVI or any other container. Film files support is being dropped in favour of SER, but simple sum stackings can still be done on them. The file extension for FITS files can be configured in the settings, by default it is .fit. Changing it will change the extension used to detect FITS sequences in the current working directory, but also the extension of the files created by Siril in conversion or any processing that creates an image.