Our documentation at Upload a file warns that we should not use special characters on the name of the file:

However, we do not validate the file name when attaching to a page, so users might upload files with special characters. If these characters are not properly encoded using the URL encoding (percent-encoding), the links to the attachments might be invalid and return HTTP error 400 (Bad Request or Not found error).

The recommended Special characters to be URL encoded (percent-encoding) are: ':', '/', '?', '#', '[', ']', '@', '!', '$', '&', "'", '(', ')', '*', '+', ',', ';', '=', as well as '%' itself. Other characters don't need to be encoded, though they could (Reference: https://developer.mozilla.org/en-US/docs/Glossary/Percent-encoding)

Testing the upload of some files using these characters, we could observe that the solution applied is following a character-per-character basis.
Some characters are URL-encoded, others are replaced when uploading or downloading, and some are URL encoded but the information retrieved by the REST API Get Page By ID shows a different encoding (see table below for some samples).

Goal: It would avoid many issues if the link always uses URL-encoding as a pattern for the attachments.

Test:

char	Attachment URL	Downloaded file	Get page by ID	notes
! (%21)	x/download/attachments/26673219/a!a.jpg?api=v2	a!a.jpg	a!a.jpg	not encoded
" (%22)	x/download/attachments/26673219/x%22x.jpg?api=v2	a_a.jpg	x%22x.jpg	replaced
"#" (%23)	x/download/attachments/26673219/a%23a.jpg?api=v2	a#a.jpg	a%23a.jpg
$ (%24)	x/download/attachments/26673219/a$a.jpg?api=v2	a$a.jpg	a$a.jpg	not encoded
% (%25)	x/download/attachments/26673219/a%25a.jpg?api=v2	a%a.jpg	a%25a.jpg
& (%26)	x/download/attachments/26673219/a%26a.jpg?api=v2	a&a.jpg	a& amp;a.jpg	CONFCLOUD-77723
' (%27)	x/download/attachments/26673219/a%27%20a.jpg?api=v2	a'a.jpg	a'a.jpg	different encoding
( (%28)	x/download/attachments/26673219/a(a)a.jpg?api=v2	a(a)a.jpg	a(a)a.jpg	not encoded
"*" (%2A)	x/download/attachments/26673219/a*a.jpg?api=v2	a_a.jpg	a*a.jpg	replaced and not encoded
+ (%2B)	x/download/attachments/26673219/a%2Ba.jpg?api=v2	a+a.jpg	a+a.jpg	different encoding
/ (%2F)	x/download/attachments/26673219/y:y.jpg?api=v2	a_a.jpg	y:y.jpg	replaced twice
: (%3A)	x/download/attachments/26673219/a:a.jpg?api=v2	a_a.jpg	a:a.jpg	replaced and not encoded
; (%3B)	x/download/attachments/26673219/a;a.jpg?api=v2	Error 400	a;a.jpg	Error 400 CONFCLOUD-74200
\ (%5C)	x/download/attachments/26673219/a%5Ca.jpg?api=v2	Error 400	a%5Ca.jpg	Error 400 CONFCLOUD-74200