Summary
After extracting some duplicate rules with mdapi command I needed to convert only one to source format. I tried to do it in several ways as you can see below. At first, I thought the problem was the destination folder, but after converting the whole duplicateRules folder, and using the same destination folder, the conversion worked. So I concluded that the cli does not work with file paths, only with folder paths.

If this is working as designed, I would like to convert it to a feature, as this is a command that Im often using. The reason I did not extracted using the source command was because I did not know the metadata name. So I had to extract using the mdapi command passing package.xml for duplicateRules and using *.
Steps To Reproduce:
Extract any metadata type
Run sfdx force:mdapi:convert -r PATH_TO_METADATA_FILE
Verify you can't convert the file
Run sfdx force:mdapi:convert -r PATH_TO_METADATA_TYPE_FOLDER
Verify you can convert the folder but not a single file
Expected result
The command should convert single files
Actual result
The command only converts files if the input is a folder
Additional information

Summary
After extracting some duplicate rules with mdapi command I needed to convert only one to source format. I tried to do it in several ways as you can see below. At first, I thought the problem was the destination folder, but after converting the whole duplicateRules folder, and using the same destination folder, the conversion worked. So I concluded that the cli does not work with file paths, only with folder paths.
If this is working as designed, I would like to convert it to a feature, as this is a command that Im often using. The reason I did not extracted using the source command was because I did not know the metadata name. So I had to extract using the mdapi command passing package.xml for duplicateRules and using *.
Steps To Reproduce:
Extract any metadata type
Run
sfdx force:mdapi:convert -r PATH_TO_METADATA_FILEVerify you can't convert the file
Run
sfdx force:mdapi:convert -r PATH_TO_METADATA_TYPE_FOLDERVerify you can convert the folder but not a single file
Expected result
The command should convert single files
Actual result
The command only converts files if the input is a folder
Additional information