Hi and welcome to inconsistency corner :grin:.

This is a problem that infests many places of both operating systems and languages and libraries, what to call stuff relating to files, common options seem to be:

* `path` or `file path` or `filename`, the location of a file eg `/home/me/mine/file.ext`

* `file` or `filename` or `basename`, the bit after the last separator in a path eg `file.ext`

* `file suffix` or `suffix` or `file extension` or `filename extension` or `extension`, the bit after (and usually including) the last dot in a `file` or `filename` or `basename` eg `.ext` (not counting weirdos like `.tar.gz`, did I mention inconsistency?) It is usually used to indicate something about the contents of the file, eg .c is C language code and .rs is Rust language code.

* `filetype` is a Geany specific term to indicate the expected contents of the file and to select the language support features of Geany, it may be selected by the extension of the `file` or specific content markers or by the user. Technically it is the `buffertype` as it relates to the buffer contents, but `filetype` has become so entrenched in the project that I doubt it can be changed.

Which words you use depends on what your programming/computer use background is, I am not sure any particular one is "correct".

To the specific examples:

Explicitly defines a filetype for the file, if it would not be detected by filename extension.

Note if you choose multiple files, they will all be opened with the chosen filetype.

I would say replace "would" by "was" and "filename" by "the" are the only changes needed if at all.

Proxy plugin '%s' extension '%s' starts with a dot. Please fix your proxy plugin.

Seems fine, the code that generates the message is indicating that in this instance the extension must be specified without the separating dot.

Path of the file representing the project and storing its settings. It should normally have the "%s" extension.

Seems fine, the `path` by definition includes the `extension`.

Unknown filetype extension for "%s".

Since %s is the name of a file probably better as "Unknown filetype for extension of %s"

Hide file extensions:

Given that this is an option to hide the extensions of a list of files its probably quite reasonable.