Fix img to pdf merge conversion type

[omar-ahmed42]

Description

Root cause:

• After decrypting the files, the operation "set" was used instead of "append" for inputs files in FormData in download.js

Solution

1. Remove fileInput entry from FormData to avoid duplicate values.
2. Append files to the FormData instead of setting them on each operation (as "set" would replace the existing values with the latest one).

Closes #2456

Checklist

• I have read the Contribution Guidelines
• I have performed a self-review of my own code
• I have attached images of the change if it is UI based
• I have commented my code, particularly in hard-to-understand areas
• If my code has heavily changed functionality I have updated relevant docs on Stirling-PDFs doc repo
• My changes generate no new warnings
• I have read the section Add New Translation Tags (for new translation tags only)

[omar-ahmed42]

Note: There's a bug in "Convert to separate PDFs", reason? form data isn't being sent in the body, only the file is being sent.
(The problem lies mainly in download.js in the frontend, not the backend)

I might work on this later, but let's roll this fix out first so that users could at least merge their images into a single PDF

[Frooodle]

Thanks for this quick find and fix, will merge and release tomorrow morning

[omar-ahmed42]

Note: There's a bug in "Convert to separate PDFs", reason? form data isn't being sent in the body, only the file is being sent.
(The problem lies mainly in download.js in the frontend, not the backend)
I might work on this later, but let's roll this fix out first so that users could at least merge their images into a single PDF

Delivered in PR #2492 as promised