Convert Blob to file

Hello, I have a blob of type webm, I need to convert it to a file, access it and in sequence convert to MP4 to save it somewhere. Can someone give me a hand with any of these steps?
Briefly, I’m putting together an application that takes 3 photos of the camera and makes a video or gif with them, I tried to transform it into a gif but I lose a lot of the quality of the images and it takes a lot of time to process on the device. Thank you for your help.