We could introduce a new function like get_acceptable_file_extension()

Should I add it to GP_Format?

Yes, I think it'd be a good place.

Yes, I think it'd be a good place.

So you suggest a new get_acceptable_file_extension() that strips any first part of the extension (eg. [jed.]json). Should this method also add the "." to the extensions?

With the exception of this list of extensions used in this PR where only the last part of the extension counts for the browser to look up, I don't see much of a use for a new method that uses the output of get_file_extensions() and strip any first part of the extension in case there are multiple parts (eg. jed.json), in the GP_Fotmat class.
Also, adding a "." is just for use in this browser lookup situation, considering all that needs to be done afterward, it doesn't make sense to me to add it in a GP_Format output.

A different option would be adding a function to misc.php where all this is done: getting all the extensions, optionally stripping the non-ending part of the extensions, removing duplicates, and adding the ".", a new gp_get_formats_extensions() that would output a simple array of all the formats extensions, with no duplicates.

@akirk here's an example of what I have in mind:

A function gp_get_format_extensions() which outputs all the formats prefixed with . delimiter, allowing multiple extensions like .jed.json.

gp_get_format_extensions():

array(
  '.xml',
  '.po',
  '.pot',
  '.mo',
  '.resx',
  '.resx.xml',
  '.strings',
  '.properties',
  '.json',
  '.jed.json',
  '.ngx.json',
)

The multiple extensions like .jed.json aren't redundant as it allows the user to select only that option, filtering out files with the single extension .json. Because of this, I think I should remove the preg_replace that strips out the multiple extensions.

Would this make sense to add to misc.php?