This is an image file created using WinImage (.ima format). This creates a duplicate of the entire diskette at the sector level. You may use WinImage to recreate the diskette, or you may also use 7-zip to read & extract the file contents. 7-zip won't create an exact duplicate of the diskette, while WinImage will. This is of no consequence in this instance, as the diskette isn't bootable.

There are other applications that can handle .ima files, such as PowerISO and UltraISO. Most people will likely want to use 7-zip over these applications as 7-zip is completely free to use.

WinImage can be found here: https://www.winimage.com

7-zip can be found here: https://www.7-zip.org