Just 3 lines of script to be able to test a droplet application without leaving applescript editor
set fich to POSIX file "/Appli/conv2spct.app" as string
tell application "Finder" to open POSIX file "/Users/yourusername/Desktop/somefile" using application file fich
If there are errors in your droplet a display dialog will be opened by the script editor applescript
Here's a pragmatic alternative using do shell script, which potentially allows you to specify multiple file arguments:
do shell script "open -a /Appli/conv2spct.app ~/Desktop/somefile1 ~/Desktop/somefile2"
The above paths happen not to need quoting (escaping) for the shell, but when using variables to specify the file paths, it's best to use quoted form of (to pass multiple arguments, apply quoted form of to each):
do shell script "open -a " & quoted form of fileappli & " " & quoted form of fileargument